[Paper Review] Multi-modal Spectroscopic Study of Surface Termination Evolution in Cr2TiC2Tx MXene

James L. Hart, Kanit Hantanasirisakul+7 more|arXiv (Cornell University)|Jan 5, 2021

MXene and MAX Phase Materials4 citations

TL;DR

This study employs multi-modal in situ spectroscopy and DFT calculations to investigate surface termination evolution in Cr2TiC2Tx MXene during vacuum annealing. It demonstrates that fluorine terminations are completely removed by 600 °C, while oxygen terminations remain stable up to at least 700 °C, enabling thermal control of surface chemistry for magnetic MXene engineering.

ABSTRACT

Control of surface functionalization of MXenes holds great potential, and in particular, may lead to tuning of magnetic and electronic order in the recently reported magnetic Cr2TiC2Tx. Here, vacuum annealing experiments of Cr2TiC2Tx are reported with in situ electron energy loss spectroscopy and novel in situ Cr K-edge extended energy loss fine structure analysis, which directly tracks the evolution of the MXene surface coordination environment. These in situ probes are accompanied by benchmarking synchrotron X-ray absorption fine structure measurements and density functional theory calculations. With the etching method used here, the MXene has an initial termination chemistry of Cr2TiC2O1.3F0.8. Annealing to 600 C results in the complete loss of -F, but -O termination is thermally stable up to (at least) 700 C. These findings demonstrate thermal control of -F termination in Cr2TiC2Tx and offer a first step towards termination engineering this MXene for magnetic applications. Moreover, this work demonstrates high energy electron spectroscopy as a powerful approach for surface characterization in 2D materials.

Motivation & Objective

• To understand the thermal evolution of surface terminations in Cr2TiC2Tx MXene, a recently discovered magnetic 2D material.
• To develop a method for controlling surface functionalization to tune magnetic and electronic properties.
• To establish high-energy electron spectroscopy as a viable tool for in situ surface characterization of 2D materials.
• To correlate experimental spectroscopic data with DFT calculations for accurate surface chemistry analysis.

Proposed method

• Conducting in situ electron energy loss spectroscopy (EELS) during vacuum annealing of Cr2TiC2Tx MXene samples.
• Applying novel in situ Cr K-edge extended energy loss fine structure (EXELFS) analysis to probe local coordination environments.
• Performing benchmark synchrotron-based X-ray absorption fine structure (XAFS) measurements for cross-validation.
• Using density functional theory (DFT) calculations to interpret experimental spectroscopic data and predict surface termination stability.
• Analyzing the evolution of surface chemistry from an initial termination of Cr2TiC2O1.3F0.8 through thermal treatment up to 700 °C.
• Combining multi-modal spectroscopic data to build a comprehensive picture of surface termination dynamics.

Experimental results

Research questions

• RQ1 How does thermal annealing affect the surface termination chemistry of Cr2TiC2Tx MXene?
• RQ2 What is the thermal stability limit of oxygen and fluorine terminations in Cr2TiC2Tx MXene?
• RQ3 Can in situ high-energy electron spectroscopy accurately track changes in surface coordination environments during thermal treatment?
• RQ4 To what extent do DFT calculations support the experimental observations of termination evolution?
• RQ5 Can thermal annealing be used as a reliable method to engineer surface terminations for magnetic applications?

Key findings

• Fluorine terminations are completely removed from Cr2TiC2Tx MXene upon annealing to 600 °C.
• Oxygen terminations remain thermally stable up to at least 700 °C, indicating high thermal resilience.
• The initial surface chemistry of Cr2TiC2Tx is confirmed as Cr2TiC2O1.3F0.8 using multi-modal spectroscopic analysis.
• In situ Cr K-edge EXELFS provides direct evidence of changes in the local coordination environment during annealing.
• High-energy electron spectroscopy proves effective for in situ surface characterization of 2D MXene materials.
• The combination of in situ EELS, XAFS, and DFT enables precise tracking of surface termination evolution under thermal treatment.