What are the basic properties of MXenes?

Basic properties of MXenes

1.Surface functional group

The surface functional groups of MXenes not only affect electrochemical performance, but also affect electronic properties (including band structure, work function) and optical properties. Although methods for precisely controlling the surface functional group species have not been found, these surface functional groups are highly dependent on the synthetic route and post-synthesis treatment.

Ti3C2TxMorphological representation

(a) a scanning electron microscope (SEM) image of the HF route Ti3C2Tx having an accordion-like structure;
(b) Scanning electron microscope (SEM) image of the HCl-LiF route Ti3C2Tx with a more compact structure;
(c) a high-angle annular dark field (HAADF) image of the multilayer Ti3C2Tx, derived from an adjacent two Ti3C2Tx monolayer ABAB stacks of the same Ti3C2TxMAX phase;
(d) an annular bright field (ABF) image combined with HAADF, revealing surface functional groups of Ti3C2Tx;
(e) A schematic diagram of the proposed atomic structure of Ti3C2Tx, wherein the surface functional group is located at a hollow position of three adjacent carbon atoms, resulting in an ABCABC stack of atomic layers;
(f) Circular dark field scanning transmission electron microscope image (ADF-STEM) of layered Ti3C2Tx nanosheets.

2. Band structure

A number of theoretical studies have been conducted to determine the electronic band structure and characteristics of the MXenes series. Most MXenes have a metal/semi-metal strip structure, while some MXene systems are semiconductor.

Band structure of semiconductor M2CT2 MXene system
 

Transition metal elements and surface functional groups significantly affect the electronic band structure that affects MXenes.

3.Work function

The functional groups on the surface of the MXenes are directly related to the work function, which is critical for electronic device applications. OH-terminal MXenes tend to have ultra-low work functions, while other functional groups always produce large work functions.

Work function calculated by MXenes

(a) Comparison of the calculated work function of the beginning of Sc and Pt and the terminal MXenes;
(b) a function of OH- and F-functionalized MXenes and O-functionalized MXenes;
(c) surface functionalization causes a function of MXenes and surface dipole moments;
(d) The electronic positioning function (ELF) contour map of Hf2C(OH)2 shows the NFE state outside MXene;
(e) The energy position relative to the lowest NFE state of the Fermi level (Ef) is a function of the work function of MXenes compared to graphene, MoS2 and BN. The solid line is in a vacuum state.