Effect of Edge Charges on Stability and Aggregation of Ti₃C₂T _z MXene Colloidal Suspensions

Herein, the stabilities of aqueous Ti₃C₂T_z (MXene) colloidal suspensions were studied as a function of pH and sodium chloride concentrations using ζ-potential and dynamic light scattering measurements. Complete sedimentation was observed when the pH was changed to 5 or 10. In the low pH regime, protons saturate the surface functional groups, rendering the ζ-potential less negative that, in turn, leads to aggregation. In the high pH regime, the ζ-potential remained constant up to a pH of almost 12. As the molarity of NaCl increases from 0 to 0.04, the ζ-potential goes from -35 to -22.5 mV. At a molarity of 0.02, sedimentation was observed. When the pH or NaCl concentration is high, sedimentation occurred, presumably, because of a reduction in the double-layer thickness. In all cases, the sediment comprised crumpled Ti₃C₂T_z flakes. After adding charged nanoparticles to the colloidal suspension, at neutral pH, subsequent transmission electron microscope micrographs showed that the negative gold nanoparticles preferred the edges, whereas the positive ones preferred the surfaces. The charge differences between the edges and faces open opportunities for direct edge or face-specific organic functionalizations, similar to work done on other two-dimensional materials.