Quantative atomic column mapping of oxygen functionalized two‐dimensional Ti3C2
MX
ene
sheets
Authors: Persson Ingemar , Palisaitis Justinas , Persson Per 
Published: December 2016

Two‐dimensional (2D) materials, in particular MXenes, are growing in interest as a result of exhibiting excellent energy storage capabilities [1]. Prone to intercalation, high surface to volume ratio as well as originating from a large family of laminar compounds, called MAX phases, offers high chemical tunability. The latter grants a high chemical versatility originating from the many possible combinations of transition metal‐carbide or transition metal‐nitride bonds, providing a wide range a tunable properties [2]. Additionally, surface functionalization, which naturally occurs in MXene synthesis, has been shown to further alter the properties of MXene [3]. MXene synthesis is performed through chemical etching of the nanolaminated MAX structure, resulting in removal of the A layer, which is separating the transition metal‐carbide/nitride layers (MX), and the result is 2D MX layers terminated by functional groups originating from of the etching agent. To date, the most frequently studied MXene is Ti
3
C
2
, and the etching agent used is HF diluted in water, this leading to OH‐ and F‐termination groups on the MXene [3]. The surface functional groups are identified as disordered after etching [4]. However, effects of thermal treatment of functionalized Ti
3
C
2
has to the best of our knowledge not been directly imaged for single sheets. In order to further understand surface functionalization on MXenes at high temperatures, which is imperative for energy storage, a thermal evolution investigation of single Ti
3
C
2
sheets was performed in an aberration corrected transmission electron microscope (TEM).

In this contribution, we present an investigation of reorganization of the surface functionalized groups on Ti
3
C
2
MXene surfaces at high temperatures, employing atomically resolved scanning TEM (STEM) and a high angle annular dark field (HAADF) detector. A powder of Ti
3
C
2
was dispersed in ethanol, ultra‐sonicated for 1 minute and subsequently filtered on a heating holder chip. Low dose STEM annealing investigations were carried out using a DENS Solution holder in the double‐corrected FEI Titan
3
60‐300 located in Linköping operating at 300 kV.

Fig. 1 shows an atomically resolved STEM image of a single Ti
3
C
2
sheet at 500 °C. A statistical analysis was performed, effectively mapping out intensities of the atomic columns, as shown in Fig. 2. A basic Rutherford model of the electron scattering was hypothesized and a Z
2
intensity dependence was calculated for the atomic columns and possible adatoms, values are shown in Fig. 3a. In Fig. 3b, a colormap is applied on the image in Fig.1, image intensities has been vacuum intensity substracted and normalized to a value defined for a pure Ti+C column. Fig.4 present a structural model of O functionalized Ti
3
C
2
MXene based on the mapping the positions corresponding to relative intensities matching 2O adatoms in the Ti+C column. It is clear from the model that O align on top of a single Ti column forming a large hexagonal lattice, as seen in Fig. 4.