Investigation of oxidation mechanism of MXene for improving oxidation stability

Abstract

MXenes, a family of 2D transition metal carbides and/or nitrides, have attracted intensive attention owing to their unique intrinsic advantages, including excellent metallic conductivity caused by excess electron density near the Fermi level (EF), hydrophilicity arising from the abundance of the surface terminal groups, and easy solution processability. Despite the properties of the remarkable materials, including high electrical conductivity and atomically thin molecular structure, MXene has not been widely applied due to its poor oxidation stability. The synthesis process using selective chemical etching in strongly acidic conditions inevitably generates an abundance of defects or vacancies at the edge and on the surface of the synthesized MXene flakes. The defective sites are vulnerable to oxidative degradation under ambient conditions or in an aqueous suspension state, which deteriorates the intrinsic properties of MXene. Understanding the oxidation reaction of aqueous Ti3C2Tx MXene suspensions is very important to foster their fundamental academic studies and design protocols to inhibit oxidation reactions. This thesis introduces the oxidation reaction mechanism and kinetics of Ti3C2Tx MXene aqueous solution under various conditions and produces effective strategies for suppressing oxidation reactions. Chapter 1 reviews the synthesis and properties of MXene, setting the stage for the experiments carried out in this thesis. Chapter 2 proposes the oxidation chemical reaction formula of aqueous Ti3C2Tx MXene suspensions based on the experimental results. In Chapter 3, the oxidation mechanism and kinetics are investigated by the oxidation of aqueous Ti3C2Tx MXene suspensions under various pHs and temperatures. Chapters 2 and 3 provided an essential factor for improving oxidation stability in Chapter 4 by providing a mechanism based on analyzing by-products and final products of oxidized Ti3C2Tx MXene suspensions and oxidation kinetics. Chapter 4 describes the oxidative stability of aqueous Ti3C2Tx MXene suspensions through sufficient interaction of catechol-functionalized compounds with the MXene surface functional groups. Chapter 4 provides perspectives for designing MXene for various applications by synthesizing the oxidation mechanism and surface treatment technology. Chapter 5 summarizes the oxidation reaction and surface functionalization techniques of aqueous Ti3C2Tx MXene suspensions, providing a perspective to consider for the design of MXene for various applications.