# NMR Spectroscopy for Studying the Selective Etching of Ti3AlC2 to Ti3C2T x MXene Using Hexafluorosilicic Acid

## TL;DR

This paper introduces a safer and faster way to study MXene etching using NMR spectroscopy and a less hazardous acid.

## Contribution

The use of 27Al NMR spectroscopy for real-time monitoring of MAX phase etching and a new etching protocol using hexafluorosilicic acid.

## Key findings

- 27Al NMR spectroscopy can detect soluble aluminum species directly from the etching supernatant.
- Hexafluorosilicic acid is a viable, less hazardous etchant for Ti3AlC2 MAX phase.
- 19F NMR spectroscopy can quantify etching extent using trifluoroacetic acid as an internal standard.

## 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 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.

27Al NMR spectroscopy is introduced as a method to detect the etching of Ti3AlC2 to Ti3C2T
x
MXene. Soluble aluminum species produced during etching were identified by sampling the etching supernatant during the reaction. Using a combination of 27Al and 19F NMR spectroscopies, a new acid etchant, hexafluorosilicic acid, was shown to be free of HF or free fluoride in quantities detectable by 19F NMR before and during the etching reaction, with H3AlF6 as the primary etching byproduct.