Pristine MXene: In Situ XRD Study of MAX Phase Etching with HCl+LiF Solution

Abstract Many applications are suggested for Ti‐MXene motivating strong interest in studies of Ti 3 C 2 T x synthesis by solution‐based methods. However, so far only ex situ studies of the synthesis are performed, mostly due to the difficulty of handling HF‐based solutions. Here the first time‐resolved in situ synchrotron radiation X‐ray Diffraction study of MXene synthesis performed using a plastic capillary‐size reaction cell directly in HF solution is reported. This study provides the first report on the structure of “pristine MXene” formed by Ti 3 AlC 2 etching with LiF+HCl. The term “pristine” refers to the MXene structure found directly in HF solution. By comparing the interlayer distances of pristine MXene (≈13.5 Å), solvent‐free Li‐intercalated MXene (≈12.2 Å), and Li‐free MXene (≈10.7 Å), it can be concluded that the width of “slit pores” formed by terminated MX layers during the Al etching does not exceed ≈3 Å. The width of these slit pores is a key factor for HF etching of Al within the interlayers. This space constraint explains the slow kinetics of MXene formation in HF‐based synthesis methods. No intermediate phases are observed, suggesting that the crystalline MXene phase is formed by the simultaneous etching of Al and termination of Ti 3 C 2 layers.