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.). Ti₃C₂T_x MXene synthesized by CoF₂/HCl etching has layered heterogeneity due to intercalated Al³⁺ and Co²⁺ that act as pillars for interlayer spacings. This study investigates the impacts of etching environments on the compositional, interfacial, structural, and thermodynamic properties of Ti₃C₂T_x MXenes. Specifically, compared with HF/HCl etching, CoF₂/HCl treatment leads to a Ti₃C₂T_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 (ΔH_f,25°C) of Ti₃C₂T_x MXenes etched with CoF₂/HCl, ΔH_f,25°C = -1891.7 ± 35.7 kJ/mol Ti₃C₂, and etched with HF/HCl, ΔH_f,25°C = -1978.2 ± 35.7 kJ/mol Ti₃C₂, 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 ΔH_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.