Transition metal carbides (MAX) and borides (MAB) are nanolamellar solids that combine ceramic hardness with metallic conductivity. Since the discovery of MXenes by Naguib et al.[1] In 2011, through selective etching of parent MAX phases, more than 30 compositions of MXenes have been reported with promising applications in energy storage and catalysis. Parallel efforts aim to predict and obtain analogous 2D MBenes from MAB phases. However, conventional wet etching methods rely on hazardous concentrated acids and bases (e.g., HF, HCl, NaOH), raising significant concerns over safety and scalability. Molten salt etching using metal halides (e.g., CuCl₂, ZnCl₂) has emerged as a safer and scalable alternative, enabling a broader range of surface terminations (-Cl, -Br, -I) that are inaccessible via HF-based routes. Moreover, the synthesis of the aforementioned 2D materials predominantly relies on powder precursors, which hinders the investigations of their fundamental properties due to limited structural control and heterogeneity.

In this thesis, we use MoAlB (MAB phase) and V₂AlC (MAX phase) single crystals and powders to explore their conversion via molten salt etching and investigate their structural, electronic, and electrochemical properties. For MoAlB, we demonstrate etching a single Al layer to obtain mm-sized Mo₂AlB₂ single crystals. Polarized Raman spectroscopy reveals vibrational anisotropy after conversion, while transport measurements show reduced resistivity in Mo₂AlB₂ (0.53 µΩ·m) compared to MoAlB (1.21 µΩ·m), consistent with density functional theory (DFT). This is particularly remarkable given the likely higher disorder character of the converted Mo₂AlB₂ crystals. These comparisons highlight the superior transport properties of Mo₂AlB₂, positioning it among the most conductive MAB phases reported to date. Powder exfoliation studies suggest a stage-wise mechanism via metastable intermediates (Mo₂AlB₂, Mo₄Al₃B₄ and 2D structure), with XRD showing interplanar expansion and SEM revealing lamellar widening.

For V₂AlC single crystals, molten salt etching led preferentially to 3D V₂C rather than 2D V₂C, followed by conversion into 3D VC. Structural analyses confirmed expanded nanolamellar features, analogous to MXenes. This transformation pathway was reproduced in V₂AlC powders under optimized conditions. Detailed characterizations revealed that we retained an expanded lamellar structure in the converted V₂C and VC phase from the original V₂AlC precursor. Detailed electrochemical characterizations were performed using a converted VC phase as a cathode material in aqueous Zn-ion batteries. It delivered a high specific capacity of 210 mAh·g⁻¹ at 5 A·g⁻¹, retaining 55% of its capacity after 2000 cycles. It shows superior cyclic stability than other V-based carbides and oxides reported in the literature.

Reference:

[1]       M. Naguib, M. Kurtoglu, V. Presser, J. Lu, J. Niu, M. Heon, L. Hultman, Y. Gogotsi, M.W. Barsoum, Two-Dimensional Nanocrystals Produced by Exfoliation of Ti3AlC2, Advanced Materials 23 (2011) 4248–4253. https://doi.org/10.1002/adma.201102306.

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