1T/2H-MoS₂ Heterostructure for Lubrication\nEnhancement and Tribo-Induced Phase Transformation into 2H-MoS₂

Abstract

Heterostructures comprising two-dimensional (2D) nanomaterials\nof dissimilar lattice constants govern the interfacial interactions,\ninduce structural strain, and influence the mechanical properties,\nresulting in a low shear strength to minimize friction. The present\nwork addresses a facile hydrothermal approach to synthesize an ammonium\nsurfactant-stabilized 1T/2H-MoS₂ heterostructure comprising\nvan der Waals interaction-driven interfacially stacked 1T metallic\nand 2H semiconducting phases of MoS₂. The high mole ratios\nof the ammonium surfactant facilitated the formation of 1T-MoS₂ beside the 2H phase, yielding the 1T/2H-MoS₂ heterostructure.\nRaman, X-ray photoelectron spectroscopy (XPS), high-resolution transmission\nelectron microscopy (HRTEM), and X-ray diffraction (XRD) analyses\nexplicitly confirmed both the 1T and 2H phases in the 1T/2H-MoS₂ heterostructures. The long alkyl chain of the ammonium surfactant\ngrafted on the surface of the 1T/2H-MoS₂ heterostructure\nextended its dispersibility in fully formulated 10W40 engine lube\noil. A minute quantity of 1T/2H-MoS₂ in engine lube oil\n(optimum dose: 0.3 mg·mL^–1) improved the lubrication\nperformance of the steel tribopair by minimizing friction (24%) and\nwear volume (76%). Raman analyses of the worn area after the lubrication\ntest with the 1T/2H-MoS₂ heterostructure revealed the tribo-induced\ntransformation of 1T into the 2H phase and formed a 2H-MoS₂-based tribo thin film on contact surfaces of steel balls for enhancement\nof lubrication properties. The significantly low weak van der Waals\ninteraction between interfacially stacked lamellae of 1T and 2H phases\nin the 1T/2H-MoS₂ heterostructure furnished a lower coefficient\nof friction than that of 2H-MoS₂ under identical tribological\nconditions. These findings promise the use of 2D heterostructures\nfor tribological applications to enhance lubrication properties.