Synthesis of Ti2B2Clx MBenes in molten salts from theoretical and experimental perspectives

Technical Summary: Synthesis of Ti2B2Clx MBenes in Molten Salts

Problem Statement
The discovery of graphene and subsequent two-dimensional (2D) materials like MXenes has driven interest in 2D transition metal borides (MBenes) for applications in energy storage and catalysis. However, the synthesis of 2D MBenes from their parent MAB phases (metal-aluminum/boron compounds) has proven significantly more challenging than the synthesis of MXenes from MAX phases. Previous attempts to selectively etch the A-element (e.g., In, Al) from MAB precursors using acids or molten salts have frequently failed, resulting in incomplete etching, structural reorganization into 3D borides, or oxidation rather than the formation of stable multilayer 2D sheets. Specifically, the selective etching of the Ti2InB2 precursor to produce a multilayer (ml) MBene had not been successfully achieved, and the thermodynamic driving forces governing these reactions remained unclear.

Methodology
The study employs an integrated experimental and computational approach to synthesize and characterize ml-Ti2B2Clx MBenes.

• Synthesis: The precursor, Ti2InB2, was synthesized via a solid-state reaction of elemental powders with excess Ti and In, followed by acid cleaning to remove impurities. Selective etching was performed using a Lewis acidic molten salt (ZnCl2) at 600 °C. The study optimized the salt-to-MAB molar ratio (4:1) and reaction time (5 hours) within sealed quartz tubes to ensure a reactive environment.
• Characterization: The resulting material was analyzed using powder X-ray diffraction (XRD), scanning electron microscopy (SEM) with energy dispersive X-ray spectroscopy (EDX), and high-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM) combined with EDX and electron energy loss spectroscopy (EELS). In situ synchrotron XRD was utilized to monitor the reaction pathway in real-time.
• Computational Modeling: Density Functional Theory (DFT) calculations were performed using the VASP package with the rev-vdW-DF2 functional to model reaction free energies. The study compared four competing scenarios: (i) A-element substitution, (ii) MBene formation, (iii) complete disintegration, and (iv) 3D boride formation. A process-specific phase diagram approach was used to determine the thermodynamic stability of these pathways.

Key Results

• Successful Synthesis: The molten salt etching of Ti2InB2 with ZnCl2 successfully removed Indium atoms completely, resulting in a multilayer Ti2B2Clx material. EDX and EELS confirmed the absence of In and the presence of Cl terminations, with a surface coverage of 1.1 < x < 1.4.
• Structural Transformation: XRD and STEM revealed an expansion of the interlayer spacing from ~7.9 Å in the precursor to ~9.5 Å in the product, consistent with the insertion of Cl terminations. The material retained a layered morphology but with an expanded c-lattice parameter (19.1 Å).
• Reaction Mechanism: In situ XRD monitoring demonstrated a direct biphasic transformation from Ti2InB2 to ml-MBene. No intermediate phases (such as Zn-intercalated species) or transient swelling were observed, distinguishing this mechanism from some MXene syntheses which often proceed through intermediate steps.
• Thermodynamic Validation: DFT calculations corroborated the experimental findings, showing that the formation of ml-Ti2B2Cl2 has a negative reaction free energy, making it the only thermodynamically viable pathway among the four scenarios considered. Conversely, A-element substitution (forming Ti2ZnB2) was predicted to be endergonic.
• Electrochemical Performance: Initial evaluation in Li-ion batteries showed a stable discharge capacity (~226 mAh g⁻¹ initially, stabilizing around 135–157 mAh g⁻¹) and coulombic efficiency approaching 100% after the first cycle. The performance was comparable to or better than certain MAX phases and other borides.
• Comparison with CuCl2: Attempts to etch Ti2InB2 using CuCl2 resulted in partial In removal but failed to produce MBenes, leading instead to structural decomposition and oxidation. This aligns with theoretical predictions that CuCl2 favors complete disintegration over MBene formation at higher salt ratios.

Significance and Claims
The authors claim that this work provides a general approach for the development of 2D materials derived from MAB phases. By combining a molten salt synthesis strategy with a predictive thermodynamic framework, the study successfully overcomes previous barriers in synthesizing MBenes. The research highlights that:

1. Thermodynamic Screening is Critical: Simple redox potential comparisons of elemental metals are insufficient for predicting MS etching outcomes in MAB phases; a comprehensive free energy analysis of competing pathways is necessary.
2. Direct Transformation: The synthesis of ml-Ti2B2Clx proceeds via a direct biphasic transformation without intermediate phases, offering a distinct mechanistic insight compared to MAX-to-MXene conversions.
3. Scalability: The process demonstrates a high conversion yield (~75%), suggesting potential for scalable production of 2D borides.
4. Framework Expansion: The theoretical framework, originally developed for MAX phases, is validated for MAB phases, offering a predictive tool for designing and understanding the formation of other 2D materials from layered compounds.

The paper concludes that this integrated strategy advances the prediction and synthesis of MBenes and provides a foundation for targeting unexplored MBenes accessible via molten salt reaction pathways.