Thermochemistry of MXenes Synthesized by CoFx/HCl Etching

Cody Blaze Cockreham

doi:10.7273/000004446

MXene is a family of nanolayered transition metal carbides and/or nitrides with great potential in various energy-related industries. MXene contains two-dimensional (2D) metal carbide layers separated by surface-bound functional groups and intercalants (water, ions, electrolyte, etc.). The planar surface area accessed between layers offers the advantage of 2D materials like MXene. The surface chemistry and intercalants are highly dependent on the synthesis environment and post-synthesis treatment. Due to large and pliant interlayer spacings, MXene readily intercalates/deintercalates ions and electrolytes. The interplays among the solid-state ceramic layers, surface chemistry, and intercalated solvents, ions, and/or particles affect the interlayer spacing, morphology, and properties of MXenes. Therefore, a fundamental understanding of the characteristics of the layered host structure, surface chemistry, and the physical and chemical natures of intercalant–MXene guest–host interactions are crucial for optimizing intercalation-based applications. This dissertation enables thermodynamic, structural, morphological, and interfacial insights into MXenes etched in a cobalt fluoride/hydrochloric acid (CoFx/HCl, where x = 2 or 3) environment.Three studies are documented. First, an etching method guided by dissolution thermodynamics is developed to synthesize a near-pure titanium carbide MXene phase, of the form Ti3C2Tx, under a unique CoFx/HCl environment by taking advantage of aluminum fluoride complexation equilibrium in the etching agent to inhibit the formation of precipitated aluminum fluoride hydrate impurity. The energetics and roles of interlayer hydration were also discussed. Subsequently, the intrinsic interfacial and structural heterogeneity of Ti3C2Tx MXene etched with CoF2/HCl is illuminated through integrated in situ thermal analysis with calorimetric, structural, and spectroscopic methods. Interestingly, clear evidence of a layered pillared heterostructure that potentially enhances ion intercalation has been identified. Finally, the studies on CoFx-etched MXenes are concluded with a calorimetric investigation into the energetic stability and interfacial complexity of Ti3C2Tx MXenes synthesized with CoF2/HCl and a hydrofluoric/hydrochloric acid mix (HF/HCl) as the etching agents. It is found that intercalated metal cations, aluminum (Al3+) and cobalt (Co2+), reduce the stability of MXenes by increasing the interlayer distance and dimensional heterogeneity and decreasing the hydration level leading to reduced interlayer van der Waals force and less favorable water–layer binding.

Thermochemistry of MXenes Synthesized by CoFx/HCl Etching

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