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The majority of etching methods for synthesizing MXenes use water as the main solvent, which in turn limits direct use of MXenes in water-sensitive applications. In this work, we show that it is possible to etch, and delaminate, MXenes in the absence of water by using organic polar solvents and ammonium dihydrogen fluoride. We also demonstrate that electrodes made from Ti 3 C 2 T z etched in propylene carbonate, resulted in Na-ion battery anodes with double the capacity to those etched in water.

MXenes have shown promise in myriad applications, such as energy storage, catalysis, EMI shielding, among many others. However, MXene oxidation in aqueous colloidal suspensions when stored in water at ambient conditions remains a challenge. It is now shown that by simply capping the edges of individual MXene flakes, Ti3C2Tz and V2CTz, by polyanions such as polyphosphates, polysilicates or polyborates, it is possible to quite significantly reduce their propensity for oxidation even when held in aerated water for weeks. This breakthrough resulted from the realization that the edges of MXene sheets are positively charged. It is thus an example of selectively functionalizing the edges differently from the MXene sheet surfaces.

Since their discovery in 2011, the 2D transition metal carbide, nitrides, and carbonitridesdubbed MXenes -have garnered a lot of worldwide interest. Given their 2D structure, surface, or termination, chemistries play a vital role in most applications. X-ray photoelectron spectroscopy, XPS, is one of the most common characterization tools for quantifying surface terminations and overall chemistry. Herein we critically review the XPS fitting models proposed for Ti 3 C 2 T z MXene in the literature and make the case that they are at best incomplete and at worst contradictory. We propose a new fitting algorithm based on all the data obtained from previously published studies. In our approach, we assign the Ti 2p peak at 455.0 eV, to the C-Ti-O\O\O and C-Ti-C octahedra. The peaks at 456.0, 457.0, 457.9, and 459.6 eV are assigned to C-Ti-O\O\F, C-Ti-O\F\F, C-Ti-F\F\F, and TiO 2-x F 2x , respectively. The first four represent possible Ti atom terminations; the last is an oxyfluoride. In our proposed model we do not distinguish between O and OH terminations in the Ti 2p spectra; we only do so in the O spectra. Lastly, we propose and recommend a method for quantifying the surface terminations in Ti 3 C 2 T z .

Herein, the stabilities of aqueous Ti 3 C 2 T z (MXene) colloidal suspensions were studied as a function of pH and sodium chloride concentrations using ς-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 ς-potential less negative that, in turn, leads to aggregation. In the high pH regime, the ς-potential remained constant up to a pH of almost 12. As the molarity of NaCl increases from 0 to 0.04, the ς-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 Ti 3 C 2 T 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 twodimensional materials.

We present a novel strategy for constructing three-dimensional (3D) porous Ti3C2Tx (MXene) networks by alkali-induced crumpling of Ti3C2Tx nanosheets. The 3D porous Ti3C2Tx networks display high capacity and outstanding rate performance as anode materials for sodium-ion batteries.