In the liquid exfoliation method first reported by Brent et al. in 2014 and modified by others, bulk black phosphorus is first ground in a mortar and pestle and then sonicated in deoxygenated, anhydrous organic liquids such as NMP under an inert atmosphere using low-power bath sonication. Suspensions are then centrifuged for 30 minutes to filter out the unexfoliated black phosphorus. Resulting 2D monolayer and few-layer phosphorene unoxidized and crystalline structure, while exposure to air oxidizes the phosphorene and produces acid.