Reliance of MXene terminating groups on various synthetic strategies and its hot electron dynamics at MXene interfaces

Abstract

MXenes are two-dimensional (2D) materials with notable properties, attracting significant attention. These materials are often synthesized with surface functionalization like -O, -OH, and -F, which regulate their properties. While experimental synthesis typically results in mixed terminations, computational studies predict pure terminations. This study provides a comprehensive overview of the characteristics and potential applications of chemical termination in MXenes. The type of terminating groups depends on the fabrication media: aqueous media yields -OH and -O terminations, alkaline media yields -F, -Cl, -Br, and -I, ammonium bifluoride etching yields -NH⁴⁺ and NH₃, and direct synthesis methods yield various terminations on demand. We also explore the role of photo-induced non-thermalized heated electrons at interfaces in plasmonic-driven chemical processes. Understanding these electron dynamics, which occur before thermalization (∼125 fs), remains challenging. Femtosecond time-resolved spectroscopy was used to study these dynamics in MXene/molecule complexes. This technique allows distinguishing between thermalized and non-thermalized electron responses. There are two non-thermalization channels: (i) rapid transport of non-heated electrons to attached molecules in less than 50 fs, and (ii) heating of adsorbed molecules due to non-thermalized electron scattering within 125 fs. These paths depend on the irradiating wavelength and the energy differential between molecules and MXene. Computational predictions of thermodynamically stable MXene compositions are essential for directing experimental goals. This work also presents a thorough computational investigation for thermodynamically stable MXenes with 11 surface terminating groups. The analysis of results investigates factors essential for assessing the thermodynamic stability of MXenes, revealing that the chemistry of MXene surface terminations is significantly crucial to hot electron dynamics and thermodynamic stability. This review also offers insights into the MXene termination process, synthesis media, and synthetic strategies, highlighting the importance of intercalation process. It also elucidates mechanisms and opportunities in hot electron theory, interfacial heat transfer, and photocatalysis.