| dc.description.abstract | MXenes are two-dimensional (2D) transition metal carbides and nitrides; they are derived from their MAX phase precursors by top-down selective acid etching. These fascinating 2D nanomaterials have a combination of functional properties suitable for a variety of applications such as batteries, supercapacitors, electromagnetic shielding, nanocomposites, and sensors. However, fabrication and usage of devices and functional coatings based on MXenes remains challenging because MXenes are prone to oxidize and degrade rapidly in aqueous and humid environments. MXenes are known reacting with water molecules resulting in the structural and chemical degradation. Differences in the oxidation rates of MXene nanosheets have been evaluated and reported in various media (air, liquid, and solid) via multiple types of measurements to assess their performance and shelf stability.
In this work, the degree of MXene oxidation is measured by the chemical composition, crystallographic structure, and electrical property changes. The oxidation rate of MXene nanosheets was observed dependent on various factors such as temperature, humidity, pH, and dispersion concentration. The oxidation rate is also determined by the chemical stoichiometry, surface functionality, and quality of MXenes and influenced by the ions and molecules present in the dispersion.
More importantly, several novel methods have been demonstrated to mitigate or eliminate the oxidation of Ti₃C₂Tₓ and Ti₂CTₓ MXene nanosheets and films. First, we discovered that “MXene antioxidants”, such as sodium L-ascorbate, ascorbic acid, citric acid, tartaric acid, and oxalic acid, can effectively extend the chemical stability of MXene nanosheets in aqueous dispersions. The success of this method is evident in the stable morphology, unchanged chemical structure, and colloidal stability of MXene nanosheets. The minimal electrical property changes also reveal that the resistance to oxidation persists in the dehydrated MXenes as well after antioxidant pretreatment. This study also focuses on the structural-activity relationship of MXene antioxidants which helps to identify the protection mechanism and to choose more effective antioxidants for MXenes. In addition to the introduction of antioxidants, we demonstrated that thermally annealing free-standing MXene films under inert argon gas can induce surface and structural modifications, which can remarkably enhance the chemical stability of the films and improve their electronic properties. These methods have potential to be also used in protecting other types of MXenes besides Ti₃C₂Tₓ and Ti₂CTₓ. Findings in this dissertation can help to solve one of the most pressing challenges in the field of MXene engineering. | |
