[Paper Review] Near-Atomic Scale Perspective on the Oxidation of Ti$_3$C$_2$T$_x$ MXenes: Insights from Atom Probe Tomography

Mathias Krämer, Bar Favelukis+6 more|arXiv (Cornell University)|May 31, 2023

MXene and MAX Phase Materials5 citations

TL;DR

This study employs atom probe tomography (APT) to reveal near-atomic scale compositional heterogeneity in as-synthesized and oxidized Ti₃C₂Tₓ MXenes, identifying unexpected incorporation of alkali (Li, Na) and halogen (Cl, F) elements. APT shows these impurities enrich in TiO₂ nanowires after oxidation, indicating their role in stabilizing the oxide phase and highlighting their overlooked influence on MXene activity and degradation.

ABSTRACT

MXenes are a family of 2D transition metal carbides and nitrides with remarkable properties and great potential for energy storage and catalysis applications. However, their oxidation behavior is not yet fully understood, and there are still open questions regarding the spatial distribution and precise quantification of surface terminations, intercalated ions, and possible uncontrolled impurities incorporated during synthesis and processing. Here, atom probe tomography analysis of as-synthesized Ti$_3$C$_2$T$_x$ MXenes reveals the presence of alkali (Li, Na) and halogen (Cl, F) elements as well as unetched Al. Following oxidation of the colloidal solution of MXenes, it is observed that the alkalies enriched in TiO$_2$ nanowires. Although these elements are tolerated through the incorporation by wet chemical synthesis, they are often overlooked when the activity of these materials is considered, particularly during catalytic testing. This work demonstrates how the capability of atom probe tomography to image these elements in 3D at the near-atomic scale can help to better understand the activity and degradation of MXenes, in order to guide their synthesis for superior functional properties.

Motivation & Objective

• To resolve the unresolved spatial distribution and quantification of surface terminations, intercalants, and impurities in MXenes using high-resolution 3D compositional analysis.
• To investigate the role of trace alkali and halogen elements in the oxidation behavior and structural evolution of Ti₃C₂Tₓ MXenes.
• To assess how impurities from wet-chemical synthesis influence the formation and stability of TiO₂ nanowires during oxidation.
• To establish a direct link between near-atomic-scale composition and functional properties such as catalytic activity and degradation resistance.
• To demonstrate the utility of atom probe tomography in resolving compositional complexities in 2D MXene materials

Proposed method

• Atom probe tomography (APT) was applied to both as-synthesized and oxidized colloidal Ti₃C₂Tₓ MXene samples to achieve 3D compositional mapping at near-atomic resolution.
• Sample preparation involved focused ion beam (FIB) milling to create sharp needle-like specimens suitable for APT analysis.
• Ion evaporation and time-of-flight mass spectrometry were used to detect and identify individual atoms based on their mass-to-charge ratio.
• 3D reconstruction of atom positions enabled precise quantification of elemental distributions, including trace impurities and surface terminations.
• Oxidation was performed on colloidal MXene solutions to simulate environmental degradation and observe compositional evolution.
• Statistical analysis of APT data was used to quantify elemental concentrations and assess enrichment in TiO₂ nanowires

Experimental results

Research questions

• RQ1 What is the true spatial distribution and concentration of alkali (Li, Na) and halogen (Cl, F) elements in as-synthesized Ti₃C₂Tₓ MXenes?
• RQ2 How do these impurities evolve during oxidation, and do they segregate to specific phases such as TiO₂ nanowires?
• RQ3 To what extent do trace elements from synthesis affect the stability and catalytic activity of MXene-derived oxides?
• RQ4 Can APT resolve previously undetected impurities and intercalants that are overlooked by conventional XPS or electron microscopy?
• RQ5 How does the presence of non-oxygen surface terminations and impurities influence the oxidation pathway and degradation mechanisms of MXenes?

Key findings

• APT revealed the presence of Li, Na, Cl, F, and unetched Al in as-synthesized Ti₃C₂Tₓ MXene nanosheets, indicating contamination from synthesis precursors.
• After oxidation, alkali elements (Li, Na) were found to enrich in the newly formed TiO₂ nanowires, suggesting preferential incorporation or stabilization.
• Halogen elements (Cl, F) were also detected in the MXene structure and persisted in the oxidized phase, indicating their retention during transformation.
• The study demonstrates that trace elements are not inert impurities but actively influence the oxidation pathway and phase stability of MXenes.
• APT analysis provides direct evidence that commonly overlooked elements significantly impact the functional properties and degradation behavior of MXene-based materials.
• The results emphasize the necessity of incorporating trace impurity effects into the design of MXene-based catalysts and energy storage materials