Atomically Resolved Structural and Chemical Investigation of Single MXene Sheets

Abstract

The properties of two dimensional (2D) materials depend strongly on the chemical and electrochemical activity of their surfaces. MXene, one of the most recent additions to 2D materials, shows great promise as an energy storage material. In the present investigation, the chemical and structural properties of individual Ti 3 C 2 MXene sheets with associated surface groups are investigated at the atomic level by aberration corrected STEM-EELS. The MXene sheets are shown to exhibit a non-uniform coverage of O-based surface groups which locally affect the chemistry. Additionally, native point defects which are proposed to affect the local surface chemistry, such as oxidized titanium adatoms (TiO x ), are identified and found to be mobile. * To whom correspondence should be addressed † Department of Physics, Chemistry and Biology, Linköping University, SE-581 83 Linköping, Sweden The latter originate from the MAX phases, consisting of a transition metal (M), an Agroup (A) element and C or N (X). 4 The significance of the MAX phases stems from their laminated structure, where M n+1 X n sheets are interleaved with atomically thin A layers. 5,6The M-X bonds consist of a mixture of covalent, metallic, and ionic bonds 6 making MXene exceptionally strong and hence appealing for new applications. 6-8MXenes are synthesized from the MAX phases by removal of A-layers through chemical etching, resulting in stand-alone 2D sheets. 5,6,8,9 is crucial to advance the understanding of MXenes. Here, we investigate the structural properties of single, double and multiple sheets of Ti 3 C 2 T x with attached surface groups by atomic-resolution scanning transmission electron microscopy (STEM). The elemental and chemical properties were further investigated by electron energy loss spectroscopy spectrum imaging (EELS SI). Through these methods we identify the atomic structure of the MXene sheets as well as intrinsic defects, surface and edge terminations by the partial coverage of surface groups. We also report on the mobility and migration of intrinsic defects and surface groups.Ti 3 C 2 T x powder was produced from Ti 3 AlC 2 which was prepared by ball-milling Ti 2 AlC (> 92 wt%, 3-ONE-2, Voorhees, NJ) and TiC (99 %, Johnson Matthey Electronic, NY) powders in a 1:1 molar ratio for 24 h using zirconia balls. The mixture was annealed at 1350• C for 2 h in argon. The sintered compact was converted to a powder by milling. Ti 3 C 2 MXene powder was prepared by immersing 2.5 g of < 400 mesh Ti 3 AlC 2 powder in 1 M of NH 4 HF 2 (Sigma Aldrich, USA) solution for 5 days. After treatment the suspension was washed several times using deionized water and centrifuged to separate the settled powder from the supernatant. The settled powders were removed from vials using ethanol and dried at room temperature. TEM samples were prepared by crushing the powder in a mortar and 3 dispersing the powder on a holey carbon Cu TEM-grid, followed by immediately insertion into the TEM.Characterization was performed using the Linköping double corrected FE...

On the Structural Stability of MXene and the Role of Transition Metal Adatoms

“…1b alone, 75% of each surface is covered by Nb adatoms and it is proposed that these are Nb adatoms that were adsorbed on the Nb 2 CT x surfaces during the applied etching, by complete or partial disintegration of adjacent MXene sheets. In strong contrast to other observations of MXenes, [34][35][36] the present example includes extensive amounts of adatoms. A high adsorption capacity of heavy metal adatoms on MXene surfaces was predicted by theoretical means (e.g.…”

“…STEM has previously been successfully implemented for atomic level investigation of MXenes. [34][35][36] The present study reports on the presence and coordination of Nb 2 C surface adsorbed species, and their influence on the structural stability over time. The results were acquired using atomically resolved STEM imaging combined with EELS and STEM image simulations.…”

Two‐Dimensional Transition Metal Carbides and Nitrides (MXenes): Synthesis, Properties, and Electrochemical Energy Storage Applications

“…The flake stacking and surface terminations of various MXenes were studied recently by electron energy‐loss spectroscopy in transmission electron microscopy (TEM), neutron scattering, and nuclear magnetic resonance (NMR) spectroscopy . These studies confirmed that there is a random distribution of MXene surface terminations, rather than having a specific type of functionalities in one region .…”

2D Metal Carbides and Nitrides (MXenes) as High‐Performance Electrode Materials for Lithium‐Based Batteries

“…[13c] Naguib et al proposed the first structural model of stacked Ti 3 C 2 (OH) 2 MXene via a density functional theory (DFT) simulation. [47] Meanwhile, Wang et al reported that the top sites of the Ti (c) atoms and the carbon atoms of the MXene monolayer are the most favorable locations for the functional groups and the intercalated sodium ions, respectively. A more precise DFT model and the ground state of multilayer Ti 3 C 2 (OH) 2 MXene have been discovered later by Mochalin and co-workers.…”