NMR Spectroscopy for Studying the Selective Etching of Ti 3 AlC 2 to Ti 3 C 2 T x MXene Using Hexafluorosilicic Acid

ABSTRACT The stringent safety protocols required for hydrofluoric acid (HF) based MAX phase etching and the reliance on material characterization tools such as XRD, SEM, EDS and XPS to study etching make the MXene research challenging. Here, we have employed 27 Al NMR spectroscopy for the rapid detection of selective etching, directly from the etching supernatant, of soluble aluminum species generated during the MAX phase etching reaction. This technique was applied to the development of a new etching protocol for Ti 3 AlC 2 MAX phase using the less hazardous hexafluorosilicic acid. The etching process was studied using a combination of 27 Al and 19 F NMR spectroscopies where it was demonstrated to be free of HF or free fluoride in quantities detectable by 19 F NMR, and that the primary etching byproduct is H 3 AlF 6 . 19 F NMR spectroscopy was additionally proven to be a viable technique to quantify the extent of etching using trifluoroacetic acid as an internal standard.