DFT-Guided Molten Salt Synthesis of Cl-Terminated Nb4C3 MXene Nanomaterial via CuCl Lewis Acid Etching for Electrochemical Applications

Abstract Conventional etching routes for Al-containing MAX phases rely primarily on fluoride-based chemistries, which provide limited control over surface terminations and motivate the development of alternative etching strategies. To address this, CuCl is evaluated as a Lewis acid molten salt etchant for Al removal from Nb4AlC3, and DFT reaction-energy calculations are used as a pre-experimental screening tool to evaluate the energetic plausibility of the proposed reaction pathway. Reaction-energy (ΔErxn) values for the selected stoichiometric steps indicate that the corresponding product configurations are lower in electronic energy than the reactant configurations within the adopted DFT model. Projected density of states (PDOS) calculations were also performed to track how the electronic structure evolves across the reaction, indicating retention of the metallic Nb−C electronic framework following Cl termination. Guided by these computational findings, Nb4C3Tx is synthesized by reacting Nb4AlC3 with CuCl at 700 °C for 8 h. X-ray diffraction (XRD) showed a shift of the (002) reflection from 7.55° to 6.55° (2θ), indicating interlayer expansion associated with Al removal and Nb4C3Tx formation. Field emission-scanning electron microscopy (SEM) imaging shows the layered nanoscale morphology characteristic of etched Nb4C3Tx, and High-resolution transmission electron microscopy (HRTEM) confirms that hexagonal structural ordering was retained, with resolved lattice fringes of 0.26 nm. The X-ray photoelectron spectroscopy (XPS) confirms the presence of Nb−C bonding environments together with Cl surface terminations consistent with the Cl-containing surface chemistry considered in the DFT model. The calculated reaction energies represent static electronic energies at approximately 0 K and do not include finite-temperature Gibbs free energies, activation barriers, or reaction kinetics at 700 °C; therefore, they provide a comparative screening criterion rather than independent proof of reaction spontaneity or the experimental mechanism. Altogether, these results demonstrate the potential of DFT-assisted energetic screening for selecting molten-salt etchants and controlling MXene surface chemistry, with relevance to electrochemical applications such as energy storage and sensing applications.