Structural-Defect-Mediated Grafting of Alkylamine on Few-Layer MoS and Its Potential for Enhancement of Tribological Properties.

Two-dimensional transition-metal dichalcogenides possess inherent structural characteristics that can be harnessed for enhancement of tribological properties by making them dispersible in lube media. Here, we present a hydrothermal approach to preparing MoS nanosheets comprising 4-10 molecular lamellae. A structural-defect-mediated route for grafting of octadecylamine (ODA) on MoS nanosheets is outlined. The unsaturated d orbitals of Mo at the sulfur vacancies on the MoS surface are coupled with the electron-rich nitrogen center of ODA and yield ODA-functionalized MoS (MoS-ODA). The MoS-ODA nanosheets exhibit good dispersibility in lube base oil and are used as an additive (optimized dose: 0.1 mg·mL) to mineral oil. It is shown that even at low concentration, MoS-ODA nanosheets significantly reduce the friction (48%) and wear (44%). Microscopy (field emission scanning electron microscopy (FESEM) and atomic force microscopy (AFM)) and spectroscopy (Raman and elemental mapping) analyses of worn scars revealed the formation of MoS-based protective thin films for lowering of friction and wear. This work, therefore, presents a pathway for low-friction lubricants by deploying functionalized low-dimensional material systems.