Facile pyrolysis synthesis of ionic liquid capped carbon dots and subsequent application as the water-based lubricant additives

Carbon dots (CDs) were facilely synthesized by one-pot pyrolyzing a lactic acid-ionic liquid (IL) gel with a relatively high yield of 34.9 wt%. The CDs, with a mean particle size of 4.4 nm and high thermal stability, were almost amorphous and capped by IL groups. The CDs-IL in water exhibited an obvious excitation-, concentration-, pH-dependent and high salt-tolerant photoluminescence property. Specifically, the CDs-IL emitted the strongest blue fluorescence at 454 nm under 365 nm excitation with a quantum yield of 16%. In addition, the CDs-IL, as a kind of water-based lubricant additives, greatly improved the tribological properties of base liquid. The loading capability of base liquid for steel–steel pair increased from 50 N to at least 80 N when only 0.015 wt% of CDs-IL was added. Meanwhile, the mean friction coefficient and wear volume of base liquid reduced by 57.5% and 64% at the load of 40 N, respectively. In addition, the friction-reducing and antiwear performance of CDs-IL was far superior to the CDs and IL, reflecting a synergetic lubrication effect between the IL groups and carbon cores. Based on the worn surface analyses, the CDs were prone to be squeezed out of the rubbing surfaces during the boundary shear friction process, while the CDs-IL were prone to quickly form ordered absorption layers on rubbing surfaces by electrostatic interaction between the IL groups and steel surface, which is of key importance for the formation of effective lubrication film to largely reduce friction and wear.