A novel preparation method for MoS₂ nanosheets with good tribology performance by the combination of expansion and freeze exfoliation

Abstract

Two-dimensional MoS₂ nanosheets were exfoliated by the combination of high temperature expansion exfoliation (MoS₂-OA) and low temperature freeze exfoliation (MoS₂-OA-N₂). The samples were verified by XRD, Raman, UV–vis, FT-IR, SEM, TEM, HRTEM and AFM. The results indicated that the MoS₂-OA-N₂ nanosheets with 5–7 layers were small size, few layers and thin thickness. The formation mechanism of the MoS₂-OA-N₂ nanosheets was proposed. Furthermore, the lubrication performances of the MoS₂-OA-N₂ nanosheets in base oil were tested on the four-ball wear machine. The average friction coefficient (COF) and average wear scar diameter (AWSD) of 0.08 wt% MoS₂-OA-N₂-based oil were about 26.46% and 44.55% lower than those of the base oil. The results exhibited that the MoS₂-OA-N₂ showed the excellent tribological properties.

Introduction

In recent years, two-dimensional materials, including MoS₂, have attracted the attention of many researchers due to the weak interaction between layers and the strong in-plane bonds [1], [2], [3] And because of the unique structural characteristics, MoS₂ nanosheets show the excellent performances in the field of catalyst [4], [5] battery [6], [7] sensor [8] nanotransistor [9] hydrogen storage [10] supercapacitor [11] lubricant [12], [13] and so on. In particular, MoS₂ has excellent lubrication properties owing to the easy sliding between layers. Recently, MoS₂ nanomaterials with different sizes and shapes have been synthesized, which have been confirmed the truth of being good lubricant additives. Yi et al. [14] have successfully prepared MoS₂ nanoparticles with three different morphologies, including flower-like, microspheres and nanosheets, through hydro/solvothermal methods. The preparetion of MoS₂ as lubricant additives in the liquid paraffin can enhance the lubricating performance. Under the same conditions, the MoS₂ nanosheets showed more wonderful lubricating properties than the other two kinds of MoS₂ nanoparticles. Rajendhran et al. [12] exfoliated the bulk MoS₂ to Ni-MoS₂ nanosheets by ultrasonication and reflux techniques. The friction results revealed that the average COF and the AWSD of 0.5 wt% Ni-MoS₂-based oil descend 40–50% and 15–20% compared to the base oil, due to its small size and surface modification behavior. Wu et al. [13] inquired into the tribological properties of MoS₂ nanosheets and zinc dialkyldithiphosphates (ZDDP) as additives in ISO VG 32 white oil by using block-on-ring tests. The results suggested that the COF and AWSD of 0.25 wt% MoS₂-based fluid were about 28.6% and 34.3% less than those of ZDDP at a high load. Although many of the above studies have proved that MoS₂ nanosheets as additives in lubricant revealed better tribological properties, the difference in the degree of improvement in friction and wear is significant. This should be attributed to the difference in the structure and quality of the MoS₂ nanosheets produced by different preparation methods.

In order to achieve the optimum tribological performance, it is necessary to directly exfoliate the multi-layered MoS₂ into few layers of MoS₂ nanosheets. So far, variously reliable exfoliation methods have been developed to prepare ultra-thin two-dimensional nanomaterials, including micromechanical stripping method [15], lithium ion intercalation method [16], liquid phase stripping method [17] and so on. For example, Krishnamoorthy et al.[15] obtained few layered MoS₂ nanosheets through a ball milling method using NMP as a solvent to exfoliate from the bulk MoS₂. Liu et al. [16] developed a simple but effective method to obtain high yield and ultrathin MoS₂ nanosheets with 4.68 nm of thickness in ethylene glycol via a hydrothermal lithium intercalation and exfoliation process. Liu et al. [17] produced MoS₂ nanosheets by a simple salt-assisted direct liquid-phase exfoliation method. Through a series of characterizations, the exfoliation efficiency of the method was improved by 73 times, and nearly 60% of MoS₂ nanosheets were single layers in the dispersion solvent. Even though these methods with the solvents were effective, the process of the removing solvents after the exfoliation was extremely challenging. It was ascribed to the high boiling point and toxicity of the organic solvent and the re-stacking of MoS₂ nanosheets after removing the surfactant-containing water [2]. Therefore, it was urgent to develop a green substitution method for preparing MoS₂ nanosheets. Li et al. [18] successfully developed a ^{water freezing expansion exfoliation} method to prepare two-dimensional nanosheets, including graphite, h-BN, MoS₂ and WS₂. However, this method waste time and need to be improved. At present, it has been proved that the expansion method can effectively exfoliate graphite to graphene [19], [20]. This method has less pollution and the as-prepared graphene has high yield and few less. Similarly, the expansion method is expected to be used for the exfoliation of the bulk MoS₂ to nanosheets.

Herein, we reported an effective exfoliation technique by the combination of expansion and freeze exfoliation method to improve the quality of MoS₂ nanosheets firstly. The as-exfoliated MoS₂ nanosheets were verified by a series of characterizations. The formation mechanism of the as-exfoliated MoS₂ nanosheets was proposed and the tribological properties of the MoS₂-OA-N₂ nanosheets in lubricating oil were further researched.