Inhibition of AlF₃·3H₂O Impurity Formation in Ti₃C₂T_x MXene Synthesis under a Unique CoF_x/HCl Etching Environment

MXenes, most commonly transition metal carbides, are a family of two-dimensional (2D) materials with promising potential in, among other applications, supercapacitors and batteries. MXenes are synthesized by etching of aluminum or gallium layers in its parent MAX phase directly by HF or by HF in situ formation using a fluoride salt and strong acid. A commonly undesired byproduct of MXene synthesis is AlF₃·3H₂O. To relieve MXenes from AlF₃·3H₂O impurity, it is important to elucidate the factors that drive its formation. Here, we dually deduce the conditions that lead to AlF₃·3H₂O formation while exploring etching with cobalt fluorides (CoF₂/CoF₃). Previously uncharacterized, etching with cobalt fluorides offers a forthright method to etch MXenes while intercalating cobalt cations. The influence of this etching environment and AlF₃·3H₂O formation on MXene's structure, morphology, and surface bonding is investigated. Ionic strength of solution used for etching is found to be a critical driving factor in the formation of AlF₃·3H₂O impurity formation. Specifically, when the ionic strength falls between ∼8.5 and 10 M, AlF₃ complexation is stable. As a result, Ti₃C₂T_x MXene phase with AlF₃·3H₂O impurity is obtained. In contrast, near-pure MXene is the only solid state product when I is smaller than 8.5 M or larger than 10 M. Hence, high purity MXene phase can be synthesized by subtle compositional tuning to manipulate the ionic strength of the etching environment.