Determining the Surface Tension of Two-Dimensional Nanosheets by a Low-Rate Advancing Contact Angle Measurement.

Because of their atomic thinness, two-dimensional (2D) nanosheets need be bound to a substrate or be dispersed in material in various applications. The surface tension (ST) of a 2D nanosheet is critical for analyzing the physicochemical interactions between 2D nanosheets and other materials. To date, the determination of the ST of 2D nanosheets has relied mainly on the contact angle (CA) method. However, because of the difficulty in measuring the thermodynamically significant Young?s CA, which is the only meaningful CA that can be used to determine the ST, significant differences exist in reported STs of 2D nanosheets. In this study, we obtained such unique Young?s CAs on graphene, boron nitride, molybdenum disulfide, and tungsten disulfide nanosheets by a low-rate advancing contact angle measurement using a rigorously designed experimental setup. By interpreting the CA with Neumann?s equation of state, we determined the STs of these four nanosheets to be 29.7 ? 0.6, 30.9 ? 0.7, 27.8 ? 0.7, and 29.1 ? 0.8 mJ/m, respectively. The surface energies of these 2D nanosheets were estimated to be in the range 95?120 mJ/m by considering the contribution of ST and surface entropy. The accuracy of these determined STs was validated by the exfoliation and dispersion of 2D nanosheets in liquids with a series of STs. The study may have important implications for understanding the physicochemical interactions between 2D nanosheets and other materials and the development of 2D nanosheet-based devices.