Energetic Stability and Interfacial Complexity of Ti 3 C 2 T x MXenes Synthesized with HF/HCl and CoF 2 /HCl as Etching Agents.

MXenes are ultra-thin two-dimensional layered early transition-metal carbides and nitrides with potential applications in various emerging technologies, such as energy storage, water purification, and catalysis. MXenes are synthesized from the parent MAX phases with different etching agents [hydrofluoric acid (HF) or fluoride salts with a strong acid] by selectively removing a more weakly bound crystalline layer of Al or Ga replaced by surface groups (-O, -F, -OH, etc.). Ti 3 C 2 T x MXene synthesized by CoF 2 /HCl etching has layered heterogeneity due to intercalated Al 3+ and Co 2+ that act as pillars for interlayer spacings. This study investigates the impacts of etching environments on the compositional, interfacial, structural, and thermodynamic properties of Ti 3 C 2 T x MXenes. Specifically, compared with HF/HCl etching, CoF 2 /HCl treatment leads to a Ti 3 C 2 T x MXene with a broader distribution of interlayer distances, increased number of intercalated cations, and decreased degree of hydration. Moreover, we determine the enthalpies of formation at 25 °C (Δ H f,25°C ) of Ti 3 C 2 T x MXenes etched with CoF 2 /HCl, Δ H f,25°C = -1891.7 ± 35.7 kJ/mol Ti 3 C 2 , and etched with HF/HCl, Δ H f,25°C = -1978.2 ± 35.7 kJ/mol Ti 3 C 2 , using high-temperature oxidation drop calorimetry. These energetic data are discussed and compared with experimentally derived and computationally predicted values to elucidate the effects of intercalants and surface groups of MXenes. We find that MXenes with intercalated metal cations have a less exothermic Δ H f,25°C from an increase in the interlayer space and dimension heterogeneity and a decrease in the degree of hydration leading to reduced layer-layer van der Waals interactions and weakened hydration effects applied on the MXene layers. The outcomes of this study further our understanding of MXene's energetic-structural-interfacial property relationships.