*4.2. Liquid-Based Synthesis*

This type of exfoliation requires the use of ultrasound technologies; it further requires immersion in solvents (such as chloroform, ethanol, tetrahydrofuran, and other halides). To obtain optimal BP nanosheets with this technique, one must a process where temperature (in some cases, around 140 ◦C) and time (h) are two critical factors. Generally, the BP is placed inside a solution with 1-methyl-2-pyrrolidone as a solvent (the method is based on the passivation of the surface); then, it is exposed to ultrasounds for up to six hours, centrifuged, and finally, dispersed in water [13]. The issue is related to the need to avoid direct contact between BP and oxygen or water as much as possible. To overcome this issue, Wang et al. [24] proposed the use of argon to remove the oxygen molecules present in the solution; this would inhibit the degradation of BP.
