Faidra Amargianou1,Tianxiao Sun1,Markus Weigand1,Tyler Mathis2,Yury Gogotsi2,Tristan Petit1
Helmholtz-Zentrum Berlin für Materialien und Energie GmbH1,Drexel University2
Faidra Amargianou1,Tianxiao Sun1,Markus Weigand1,Tyler Mathis2,Yury Gogotsi2,Tristan Petit1
Helmholtz-Zentrum Berlin für Materialien und Energie GmbH1,Drexel University2
MXenes include transition metal carbides/nitrides with intercalated cations, such as Li-Ti<sub>3</sub>C<sub>2</sub>T<i><sub>x</sub></i> and K-Ti<sub>3</sub>C<sub>2</sub>T<i><sub>x</sub></i>, where T<i><sub>x</sub></i> represents surface terminations (typically O, OH and F). They have shown promising performance for energy storage, which is highly dependent on the surface chemistry and the Ti oxidation state. Nevertheless, contributions of different oxygen bonds in the local chemistry of MXene flakes and interlayers are difficult to disentangle without high chemical and spatial sensitivity.<br/><br/>In this presentation, we will show that Scanning Transmission X-ray Microscopy (STXM) is well suited for chemical imaging on individual MXenes particles. This synchrotron-based method allows the recording of soft X-ray Absorption Spectra (sXAS) with <60 nm spatial resolution, hence enabling the characterization of single MXene particles with high chemical sensitivity to Ti oxidation state and oxygen bonding, as well as bulk sensitivity. We will present here the first STXM characterization of Ti<sub>3</sub>C<sub>2</sub>T<sub>x</sub> , Li -Ti<sub>3</sub>C<sub>2</sub>T<sub>x</sub>, and K-Ti<sub>3</sub>C<sub>2</sub>T<sub>x</sub> MXenes at the Ti L-, C K- and O K-edges. The data analysis with unsupervised machine learning algorithms of STXM measurements for intercalated MXenes achieved the identification of different oxygen-containing compounds. Moreover, STXM measurements in transmission and total electron yield detection mode at Ti L- and O K-edge allowed the comparison between surface and bulk properties for Ti<sub>3</sub>C<sub>2</sub>T<sub>x</sub> MXene.<br/><br/>A decisive step towards understanding the migration of oxygen as a termination and water species was also achieved with <i>in situ</i> temperature-dependent STXM measurements for these MXenes. The local behaviour of oxygen during annealing up to 650°C was mapped at the Ti L- and O K-edge. These are the first bulk-sensitive measurements able to map oxygen redistribution inside MXene interlayer during annealing. The aforementioned experimental results advocate for the importance of spaces between MXene layers and overlapping flakes in the formation and control of oxygen-containing species in MXenes.