The Morphology of TiiAlC (M2AX) and TiiC (MXene) Sheets Revealed by HAADF STEM Analysis

Abstract

The MAX phases are a family of materials composed of a transition metals (M), an A group element (A), and carbon and/or nitrogen (X) with a combination of metallic and ceramic properties [1]. The significance of the MAX phases stems from their laminated structure, where Mn+1Xn sheets are interleaved with atomically thin A layers. The M-X bonds consist of a mixture of covalent, metallic, and ionic bonds making M-X compounds exceptionally strong and hence appealing for new applications [2]. The family of graphene-analogous material, known as MXenes, was discovered recently by Barsoum´s group [3]. The MXenes are synthesized from the MAX phases by removal of A-layers through chemical etching, resulting in stand-alone 2D sheets. It was confirmed that the properties of the MXene layered compounds such as wetting, electrical and electrochemical properties can be tunable through selections of surface functional groups. As modified surfaces make MXenes interesting in multiple fields, for instance in energy storage systems and catalysis.In this study, the structural properties of individual Ti2C (MXene) sheets are investigated at the atomic level by STEM-EELS and EDX. The Ti2C compound was generated through selective etching of aluminum (Al) from of the Ti2AlC ternary M2AX. Ti2AlC was prepared by ball-milling of Ti:Al:TiC reagents in ratio 1:1.1:1 for 24 h using zirconia balls. The mixture was annealed at 1300°C for 2 hrs in argon atmosphere. The sintered compact was converted to a powder by milling. Ti2C MXene powder was prepared by immersing Ti2AlC powder in 1M of NH4HF2 solution for 2 days [4]. After treatment the suspension was washed several times using deionized water. TEM samples were prepared by crushing the powder and next dispersion in ethanol solution, treated in ultrasound for 5 min. A drop of suspension was placed on a Ni grid. The characterization was performed using the UIC microscope JEOL JEM-ARM200CF TEM/STEM with cold field emission source; CEOS probe aberration corrector, Gatan digital cameras, Oxford Energy Dispersive X-ray Spectroscopy (XEDS) and Gatan Electron Energy Loss Spectroscopy (EELS). EELS intensities were normalized with respect to the Ti peak.From the observations of lattice fringes, solid state obtained sample revealed to have well-crystallized M2AX-phase Ti2AlC structure (Fig.1a). The d=3.3Å of the (10-10) planes were perfectly matched with those of the reported Ti2AlC [5]. M2AX phase Ti2AlC possessed a hexagonal structure in which covalent Ti2C layers are intercalated with metallic Al layers (Fig.1b). According our STEM experiment the molar ratio of Ti:Al:TiC = 1:1.1:1 led to a stoichiometric Ti2AlC phase (Fig.1d). Fig.2a shows the HAADF STEM of Ti2AlC phase etched in NH4HF2 results for Ti2C sheets. It is found atomic Ti-C columns (Fig.2b) to be imaged even for single layer structures. The d=2.0Å of the (1-100) planes were matched with those of Ti2C. The layer stacking sequence of Ti atoms along the [0 0 0 1] is visible. In conclusion, we confirm that Al containing M2AX phases could be s...