Ordered Double Transition Metal MXenes

Abstract

Since the discovery of MXenes, research has progressed tremendously to explore MXenes with new compositions apart from the conventional Ti3C2Tx. Recently, the MXene family further expanded with the discovery of ordered double transition metal MXenes, wherein the metal sites are occupied by two different transition metals, which can be arranged in‐plane or out‐of‐plane forming i‐MXenes or o‐MXenes, respectively. The feature of optimizing the properties of ordered double transition metal MXenes by precise engineering of the composition, number of metal layers, interlayer spacing, and surface functionalities is distinctive among the existing 2D materials. This review provides a brief overview of the theoretical and experimental studies on the ordered double transition metal MXenes to elucidate its structure and properties. In addition, the recent trends in the synthesis and the effects of fine‐tuning the composition, structure, and functional groups on their electrochemical performance are elaborated. The current challenges faced by these emerging MXenes and future research directions are also proposed.

Recent advances in MXenes: beyond Ti-only systems

“…46,47 When the two metals are distributed at specic sites in the M layers, they form ordered DTM MXenes, which are further classied as in-plane (i-MXene) and out-of-plane (o-MXenes) ordered DTM MXenes. 46,48,49 In an i-MXene represented as (M ′ 4/3 M ′′ 2/3 )XT x , the two metals are ordered in alternating sites of the same M layer, whereas in the case of o-MXenes represented as (M ′ 2 M ′′ ) X 2 T x and (M ′ 2 M ′′ 2 )X 3 T x , the two metals M ′′ and M ′ are ordered in two separate atomic planes, with M ′′ and M ′ atoms forming the inner and the outer layers, respectively. 48,50,51 Furthermore, when the M ′′ layer is etched along with the A layers in the i-MAX phase, they form divacancy-ordered MXenes represented as M ′ 4/3 XT x .…”

Synergistic MXene/LDH heterostructures with extensive interfacing as emerging energy conversion and storage materials

“…Double transition metal MXenes (DTM MXenes) are realized when two different metal elements (M ′ and M ′′ ) replace the M layer of pristine MXenes, changing the chemical formula to (M ′ M ′′ ) n+1 X n T x . [69][70][71][72][73][74] DTM MXenes are categorized into two groups based on the metal atom arrangements within the MXene lattices: disordered (or solid-solution) DTM MXenes and ordered DTM MXenes. The former type comprises two metal elements randomly arranged in the M layer without a well-dened stoichiometric constant, while the latter type possesses a non-metal atomic layer sandwiched between two distinct metal layers.…”

Bare W-based MXenes (WCrC and MoWC) anode with high specific capacity for Li and Mg-ion batteries

“…One kind of the 2D nanomaterials MXenes are suitable electrode because of their interesting physicochemical properties, including a high specific surface area, flexible planar structure offering multiple anchoring sites, robust mechanical structures, and tunable electronic states [5,[7][8][9][10][11][12][13][14][15][16]. However, single transition metal MXenes (STMs) face some problems, such as high molar weight, which leads to low energy density, and many MXenes with early transition metals are not metallic and are not suitable as anode materials [17]. The properties of MXenes materials can be regulated by modifying their terminations.…”