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Abstract
The secret of Van der Waals materials and 2D materials lies in their layers. Layered transition metal carbides and nitrides (MXenes) are a family of rapidly developing materials that hold great promise for interdisciplinary research. We believe that a synergistic contribution from branches of chemistry will greatly accelerate the development of interfacial platforms like MXenes. Advancing MXene synthesis is currently the key step in promoting sustainable MXene research. The currently dominant top-down etching approach benefits from its simple protocols and scalability but inevitably faces challenges in sample quality and reproducibility. Direct bottom-up approaches are expected to address the issues and advance MXene science by providing high-quality samples for fundamental physics and cutting-edge research. In Chapter 2, we introduced the first direct synthesis of MXenes. We show that Ti2CCl2 and Ti2NCl2 MXenes can be directly synthesized from Ti metal, titanium chlorides (TiCl3 or TiCl4), and various X sources, including graphite, CH4, or N2. Besides convenience and scalability, the direct synthesis routes offer synthetic modalities complementary to traditional top-down methods. For example, we demonstrate chemical vapor deposition (CVD) synthesis of extended carpets of MXene sheets oriented perpendicular to the substrate. Such orientations make MXene surfaces easily accessible for ion intercalation and (electro)chemical transformations by exposing edge sites with high catalytic activity. We later generalized the direct MXene synthesis. In Chapter 3, we used organohalides, a family of halogenated hydrocarbons (e.g., C2Cl4, CH2Cl2, and CH2Br2), as general precursors that can be combined with different transition metals to produce various MXenes (Ti2CCl2, Ti2CBr2, Zr2CCl2, Zr2CBr2, Nb2CCl2), including a new Nb2CBr2 MXene phase not accessed by other routes. These precursors can be easily handled under ambient conditions because of their stability and low corrosivity. We systematically studied the mechanism and thermodynamics of the reactions, so that the synthesis of MXenes became predictable and programmable. The use of molecular precursors enables precise control of their reactivity, which allows the direct synthesis of solution-processable MXene nanostructures. We demonstrate that nanometer-scale MXenes show higher surface reactivity compared to MXenes with micron-sized flakes. After achieving a breakthrough in MXene synthesis, in Chapter 4 we switched our focus to another unique dimension of MXene research – surface chemistry. MXenes are naturally capped with surface terminal groups that can be replaced, grafted or eliminated via post-synthesis modification. The wide chemical tunability of MXene surfaces makes them ideal modular platforms for catalysis. However, the investigation of MXene surfaces as catalytic platforms is limited. We introduce novel organometallic MXenes synthesized via electrochemical surface alkylation. These newly formed organometallic MXenes bridge solid-state materials with classic organometallic molecular motifs. In line with their unique structure, these organometallic MXenes display fundamentally new reactivity; alkyl-terminated Nb2C MXenes are catalysts for C–C activation and C≡C deletion reactions of terminal alkynes. The novel structure and reactivity of these new organometallic species, combined with the innate structural and compositional tunability of MXenes, make them a unique and emerging catalytic platform.