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.