Large-scale liquid exfoliation of two-dimensional materials such as molybdenum disulfide, tungsten disulfide, and graphene for the synthesis of printable inks is still inefficient due to many hours of exfoliation time needed to achieve a highly concentrated dispersion that is useful for printing. Here, we report that soaking the bulk 2D material powders in a variety of solvents (water, ethanol, isopropanol, acetone, methanol, dimethylformamide, N-methyl pyrrolidone, and hexane) briefly as short as 5 min "activates" them to be much more easily exfoliated afterward. The unsoaked powder yielded a negligible concentration of dispersed nanosheets (less than 0.01 mg/mL) even after long hours of sonication, while the powders soaked in water resulted in dispersed nanosheets of 1.21 mg/mL for MoS2 and 1.28 mg/mL for WS2 after 6 and 4 h of sonication, respectively, a more than 100 time increase. For graphene, soaking in methanol for 5 min prior to sonication for 6 h yielded an increase in the dispersed nanosheet concentration to 0.13 mg/mL, a more than 10 time increase in concentration. The enhanced exfoliation is originated not from the intercalated solvent molecules but from the slightly increased d-spacing of the bulk powders during soaking due to the different dielectric environments in the solvents, which assists in the exfoliation afterward. We further fabricated MoS2 and WS2 photodetectors with graphene as electrodes by one-step electrohydrodynamic (EHD) printing using highly concentrated inks (>2 mg/mL) obtained by ultrafast liquid exfoliation, which have light sensitivity down to 0.05 sun. We believe that this ultrafast exfoliation technique combined with the one-step device printing technique enables a big step toward the mass production of functional devices fabricated from solution-processed 2D material inks.
Keywords: 2D materials; electrohydrodynamic printing; exfoliation; photodetector; solvent.