NMR Spectroscopy for Studying the Selective Etching of Ti₃AlC₂ to Ti₃C₂T_x MXene Using Hexafluorosilicic Acid

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 ²⁷Al 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 Ti₃AlC₂ MAX phase using the less hazardous hexafluorosilicic acid. The etching process was studied using a combination of ²⁷Al and ¹⁹F NMR spectroscopies where it was demonstrated to be free of HF or free fluoride in quantities detectable by ¹⁹F NMR, and that the primary etching byproduct is H₃AlF₆. ¹⁹F NMR spectroscopy was additionally proven to be a viable technique to quantify the extent of etching using trifluoroacetic acid as an internal standard.