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

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