Surface Functionalization of WS2 Nanosheets with Alkyl Chains for Enhancement of Dispersion Stability and Tribological Properties.

Tungsten disulfide (WS2) exhibits intriguing tribological properties and has been explored as an excellent lubricious material in thin-film and solid lubricants. However, the poor dispersibility of WS2 has been a major challenge for its utilization in liquid lubricant applications. Herein, a top-down integrated approach is presented to synthesize oxygenated WS2 (WS2-O) nanosheets via strong acid-mediated oxidation and ultrasound-assisted exfoliation. The ultrathin sheets of WS2-O, comprising 4-7 molecular lamellae, exhibit oxygen/hydroxyl functionalities. The organosilanes having variable surface-active leaving groups (chloro and ethoxy) are covalently grafted, targeting the hydroxyl/oxygen functionalities on the surface of WS2-O nanosheets. The grafting of organosilanes is governed by the reactivity of chloro and ethoxy leaving groups. The DFT calculations further support the covalent interaction between the WS2-O nanosheets and organosilanes. The alkyl chain-functionalized WS2-O nanosheets displayed excellent dispersibility in mineral lube base oil. A minute dose of chemically functionalized-WS2 (0.2 mg.mL-1) notably enhanced the tribological properties of mineral lube oil by reducing the friction coefficient (52%) and wear volume (79%) for a steel tribopair. Raman analysis of worn surfaces revealed WS2-derived lubricious thin film formation. The improved tribological properties are attributed to ultralow thickness, stable dispersion, and low shear strength of chemically functionalized WS2 nanosheets, along with protective thin film formation over the contact interfaces of a steel tribopair. The present work opens a new avenue toward exploiting low-dimensional nanosheets for minimizing energy losses due to high friction.