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

Area of Science:

• Materials Science
• Nanotechnology
• Chemistry

Background:

• Ti3C2Tx (MXene) synthesis is of great interest for various applications.
• Current synthesis studies are limited to ex situ methods due to challenges with hydrofluoric acid (HF) solutions.

Purpose of the Study:

• To perform the first time-resolved in situ synchrotron radiation X-ray Diffraction (XRD) study of MXene synthesis directly in HF solution.
• To investigate the structure of "pristine MXene" formed during the etching process.

Main Methods:

• Utilized a plastic capillary-size reaction cell for in situ measurements directly within HF solution.
• Employed synchrotron radiation X-ray Diffraction (XRD) for time-resolved structural analysis.
• Compared interlayer distances of pristine MXene, Li-intercalated MXene, and Li-free MXene.

Main Results:

• Reported the structure of "pristine MXene" formed by etching Ti3AlC2 with LiF+HCl.
• Determined the interlayer distances: pristine MXene (≈13.5 Å), Li-intercalated MXene (≈12.2 Å), and Li-free MXene (≈10.7 Å).
• Concluded that the "slit pore" width formed during Al etching is approximately 3 Å, explaining the slow kinetics of MXene formation.

Conclusions:

• The ≈3 Å slit pore width is critical for HF etching of Al within Ti3AlC2 interlayers.
• The observed space constraint explains the slow kinetics of MXene formation in HF-based methods.
• Crystalline MXene phase formation occurs via simultaneous Al etching and Ti3C2 layer termination, with no intermediate phases observed.