Development of Safer and Scalable Methods for Etching MAX Phases to MXenes

Abstract

MXenes are two-dimensional nanomaterials that have garnered immense attention since their discovery in 2011. These transition metal carbides/nitrides are typically synthesized by selectively etching the “A” element from their precursor MAX phases using topdown chemical etching. MXene properties such as high electrical conductivity, hydrophilicity, etc., make them suitable for a wide range of applications in areas including energy storage, electromagnetic interference shielding, catalysis, and many more. Therefore, the large-scale synthesis of MXenes is required to foster their industrial applications. However, their large-scale synthesis is currently hindered due to a lack of (i) understanding of the etching mechanism of precursor MAX phases to MXenes; (ii) safer (non-toxic) and scalable etching routes for MXene synthesis at room temperature; (iii) synthesis routes that reduce the extent of MXene degradation over time; (iv) novel MAX phases designed for their scalable etching to novel MXenes. This work addresses the above bottlenecks by improving the mechanistic understanding of the MXene synthesis process, fine-tuning experimental process parameters and developing new synthesis routes and MXene compositions to enable large-scale production of MXenes. This dissertation begins with using V2CTz as a model system to develop a core-shell mechanism of etching MAX phases to MXenes using hydrofluoric acid. According to this mechanism, etching is not just limited to the removal of the Al layer from the parent MAX phase, but also involves subsequent over-etching and degradation of MX layers to carbon and vanadium oxide. In subsequent chapters, a novel and non-toxic etching route consisting of simultaneous etching and intercalation is developed for scalable MXene synthesis at room temperature using quaternary ammonium fluorides. This new method is also demonstrated to improve the oxidation-induced degradation and chemical stability of Ti3C2Tz MXene dispersions. In addition, four novel compositions of V and Ti-based solid-solution (Vx,Ti1-x)3C2Tz MXenes are discovered in this dissertation, with (V0.9,Ti0.1)3C2Tz being the highest V containing “312” MXene reported to date. Moreover, the scalability of (Vx,Ti1-x)3C2Tz etching using relatively safer 12M HF-HCl-based etchants is also investigated. The results and discussions presented in this work answer essential research questions on scalable and safer MXene synthesis faced by the MXene community.