Sanghyun Lee

According to your method, you do not control the formation of a STAB monolayer on the surface. For this, there is the Langmuir-Blodget technique and the Langmuir surface balance. On the surface of water or sodium bromide solution, you form a monolayer of STAB in a Langmuir balance. Then put the MoS2 in there and gradually remove it. Since MoS2 is hydrophobic, the STAB monolayer will be adsorbed by the hydrocarbon group on MoS2, and there will be a positive charge on the surface.

Article Piezoelectric Properties of Barium Titanate Langmuir Films

Dear Sanghyun Lee that's a very interesting technical question. As a synthetic chemist I'm not an expert in nanoparticles. However, I just came adross this very interesting research article in which the authors report that "Sonication of MoS2 powder in the presence of the cationic surfactant cetyltrimethylammonium bromide (CTAB) provided dispersions that were stable over weeks."

Water Dispersible, Positively and Negatively Charged MoS2 Nanosheets: Surface Chemistry and the Role of Surfactant Binding

Article Water Dispersible, Positive and Negatively Charged MoS 2 Nan...

Unfortunately this paper has not yet been posted on RG as public full text. Perhaps you can access it though your institution. Only the Supplementary Information is freely available as pdf file (see attached).

unfortunately, I can guess only and would try following options

#1 wash on filter until all excess CTAB and Bromide is removed - try again

#1 instead of Cetyl use dodecyl TAB