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 Ti₃C₂T_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 Ti₃AlC₂ 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 Ti₃C₂ layers.

Keywords: 2D materials; MXene; Ti3C2Tx; X‐ray diffraction; in situ synthesis; synchrotron radiation.

Figure 1

Analysis of XRD data recorded during MXene synthesis at different temperatures. a) Selected XRD patterns recorded in situ from mixed powders of precursors MAX phase and LiF (1:1) after adding HCl (λ = 0.3543 Å). b) “Heat map” showing full range of recorded diffraction angles with break excluding the region of strong reflections from plastic sample holder. c) Integral intensity ratio for (001) MXene/(002) MAX phase reflections as a function of time and temperature. Dashed lines show the moments of temperature change. d) Time dependence of d(001) value for (001) MXene reflection.

Figure 2

XRD patterns recorded from a sample obtained in the experiment shown in Figure 3 after the replacement of acidic solution with water followed by air drying (λ = 0.3543 Å).

Figure 3

Two experiments with MAX phase etching performed at 323 K. a) MXene d(001) as a function of time after adding HCl. Starting from ≈400 min, effects related to insufficient amount of HF in the solution start to be evident. Refill with HCl was done after 600 min. b) Integral intensity ratio for (001) MXene /(002) MAX reflections versus time for the experiment shown in a). c) MXene d(001) as a function of time‐showing points (dashed lines) when more HCl was added. d) Integral intensity ratio for (001) MXene/(002) MAX reflections versus time for the experiment shown in panel c).

Figure 4

XRD data recorded during the prolonged etching experiment with several refills of reaction cells with fresh HCl. a) Time‐resolved map of XRD patterns recorded during MAX phase etching at 323 K showing time for HCl refills (λ = 0.3543 Å). b) XRD patterns recorded 2 min and 560 min after adding HCl. XRD pattern of material‐free PE capillary with PP mesh is shown as a reference. All reflections in 2 min pattern are indexed according to the composition of the initial powder mixture as LiF (shown by arrows) and Ti₃AlC₂. After 560 min the XRD pattern shows (001)‐reflection from MXene. Two very broad background features found in the angle region ≈5–10 degrees are due to the liquid phase.

Figure 5

Scheme illustrating the mechanism of MXene formation by reaction of MAX phase with LiF + HCl solution etching away Al layer. a) MAX phase before Al etching by in situ formed HF and b) intercalated Ti₃C₂T_x MXene.

Figure 6

The scheme of the experimental cell is designed for in situ experiments with MXene synthesis.