Pure oxygen terminated MXenes prepared by NaOH/NaI eutectic molten salt

Abstract

MXenes, a promising class of two-dimensional materials derived from transition metal carbides and nitrides, are attracting considerable attention for their remarkable properties and potential uses in energy storage, EMI shielding, electronics, etc. Modifying surface terminations is essential for tuning the electron structure of MXenes, significantly affecting their properties. However, existing methods for altering the surface chemistry, particularly regarding oxygen termination, encounter significant challenges, such as low substitution efficiency and self-oxidation during treatment, resulting in the absence of pure O-terminated MXenes in the literature. Here, we present a general NaOH/NaI molten salt modification strategy that successfully prepares pure O-terminated MXenes. The effectiveness of this reaction stems from the naked OH ions in the molten salt, along with elevated temperatures that facilitate substitution, all while remaining below the temperatures that cause structural degradation and self-oxidation of MXenes. Consequently, the intrinsic properties of pure O-terminated MXenes undergo significant changes, including continuous tunability of the work function (3.67 to 4.17 eV) and distinct temperature-dependent resistance behavior. In addition, pure O-terminated Ti₃C₂ demonstrates impressive performance for Li⁺ storage, achieving a high specific capacity of 258 mAh g⁻¹ at 0.1 A g⁻¹, which is more than two times that of pristine Ti₃C₂T_x. In addition, after 5000 cycles at a current density of 2 A g⁻¹, the capacity retention of pure O-terminated Ti₃C₂ is 97%. Our results not only demonstrate the possibility of obtaining pure O-terminated MXenes but also validate the importance of pure O terminations on tuning the properties and boosting the performance of MXenes in various fields.