On the organization and thermal behavior of functional groups on Ti3C2 MXene surfaces in vacuum

Ingemar Persson,Per O Å Persson,Lars-Åke Näslund,Joseph Halim,Michel W Barsoum,Johanna Rosen,Vanya Darakchieva,Justinas Palisaitis

doi:10.1088/2053-1583/aa89cd

Ingemar Persson, Per O Å Persson + Show 6 more

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https://doi.org/10.1088/2053-1583/aa89cd

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Abstract

The two-dimensional (2D) MXene Ti3C2Tx is functionalized by surface groups (Tx) that determine its surface properties for, e.g. electrochemical applications. The coordination and thermal properties of these surface groups has, to date, not been investigated at the atomic level, despite strong variations in the MXene properties that are predicted from different coordinations and from the identity of the functional groups. To alleviate this deficiency, and to characterize the functionalized surfaces of single MXene sheets, the present investigation combines atomically resolved in situ heating in a scanning transmission electron microscope (STEM) and STEM simulations with temperature-programmed x-ray photoelectron spectroscopy (TP-XPS) in the room temperature to 750 °C range. Using these techniques, we follow the surface group coordination at the atomic level. It is concluded that the F and O atoms compete for the DFT-predicted thermodynamically preferred site and that at room temperature that site is mostly occupied by F. At higher temperatures, F desorbs and is replaced by O. Depending on the O/F ratio, the surface bare MXene is exposed as F desorbs, which enables a route for tailored surface functionalization.

Highlights

Since the discovery of graphene [1], two-dimensional (2D) materials have attracted vast attention as a consequence of their exceptional properties that arise from their reduced dimensionality as compared to their parent bulk materials
After the Ti3AlC2 thin film was removed from the PVD system it was converted into Ti3C2Tx by etching in 10% concentrated HF(aq) (Sigma Aldrich, Stockholm, Sweden) for 1 h at room temperature (RT)
Based on the results shown and in total agreement with the finding that at the highest temperatures the O occupies the A-sites, we can more confidently propose the peak at 529.9 eV to atomic O bridging two Ti sites on the MXene surface and not a TiO2 surface

Summary

Introduction

Since the discovery of graphene [1], two-dimensional (2D) materials have attracted vast attention as a consequence of their exceptional properties that arise from their reduced dimensionality as compared to their parent bulk materials. MXenes have been described as 2D-conductive clays [5] synthesized by exfoliation of the MAX phases, which are nanolaminated phases of carbides and/or nitrides following the general formula Mn+1AXn (n = 1, 2 or 3), where M is an early transition metal, A is a group A element, and X is either C or N [6]. To produce MXene, the MAX phase is chemically etched, resulting in the selective etching of the A layers and their replacement with surface termination. Their proper designation is Ti3C2Tx, where Tx consist of surface termination groups that include O, OH and F [7]. The resulting multilayer particles are weakly bound together and can subsequently be separated resulting in 2D metallic-like Mn+1XnTx sheets

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