Sodium hydroxide and vacuum annealing modifications of the surface terminations of a Ti ₃C ₂ (MXene) epitaxial thin film†

Abstract

We investigate, and quantify, changes in structure and surface terminations of epitaxial thin films of titanium carbide (Ti ₃C ₂) MXene, when treated by sodium hydroxide solution followed by vacuum annealing at 550 °C. Using X-ray photoelectron spectroscopy and scanning transmission electron microscopy, we show that NaOH treatment produce an increase in the c-lattice parameter together with an increase in the O terminations and a decrease in the F terminations. There is also an increase in the percentage of the binding energy of Ti-species in Ti 2p XPS region, which suggests an increase in the overall oxidation state of Ti. After subsequent annealing, the c-lattice parameter is slightly reduced, the overall oxidation state of Ti is decreased, and the F surface terminations are further diminished, leaving a surface with predominantly O as the surface terminating species. It is important to note that NaOH treatment facilitates removal of F at lower annealing temperatures than previously reported, which in turn is important for the range of attainable properties.

Abstract

Ti ₃C ₂T _x (MXene) thin film shows elimination of –F and domination of –O surface terminations after NaOH treatment followed by annealing while preserving the electrical conductivity of the film.