  Control of surface functionalization of MXenes holds great potential, and in
particular, may lead to tuning of magnetic and electronic order in the recently
reported magnetic Cr2TiC2Tx. Here, vacuum annealing experiments of Cr2TiC2Tx
are reported with in situ electron energy loss spectroscopy and novel in situ
Cr K-edge extended energy loss fine structure analysis, which directly tracks
the evolution of the MXene surface coordination environment. These in situ
probes are accompanied by benchmarking synchrotron X-ray absorption fine
structure measurements and density functional theory calculations. With the
etching method used here, the MXene has an initial termination chemistry of
Cr2TiC2O1.3F0.8. Annealing to 600 C results in the complete loss of -F, but -O
termination is thermally stable up to (at least) 700 C. These findings
demonstrate thermal control of -F termination in Cr2TiC2Tx and offer a first
step towards termination engineering this MXene for magnetic applications.
Moreover, this work demonstrates high energy electron spectroscopy as a
powerful approach for surface characterization in 2D materials.
