Concerning MXene's structural types, I'm getting stuck. According to my knowledge, the MXene structure is classified into three categories: mono transition metal, double transition metal, and divacancy transition metal.
So, I want to know whether divacancy transition mxene is considered a sort of double transition metal or a distinct subtype of mxene.
Do "in-order" and "out-order" MXene fall under the category of Double Transition Metal MXene or Divacancy MXene?
Generally, with MXenes you can either have one or more transition metals. For the single transition metal MXenes (which are the most common), you will have some of the well known structures. Ti3AlC2 leads to Ti3C2Tx, V4AlC3 leads to V4C3Tx, etc. These are the most simple structures.
When you have two transition metals, there are broadly two classes of structures that can be observed: Ordered or solid solution MXenes. For solid solution MXenes (Article Tailoring Electronic and Optical Properties of MXenes throug...
) you will have the transition metals be randomly mixed on the MXene structure. These can be M2XTx, M3X2Tx, M4X3Tx, or M5X4Tx (interesting, M5X4Tx currently only exist as solid-solutions:Article Synthesis of Mo4VAlC4 MAX Phase and Two-Dimensional Mo4VC4 M...
There is also a recent new class of MXenes "high entropy" which follow the sample structure, but with more than two elements.
For the ordered structures, you have generally two classes. For the in-plane ordered (or i-MXenes) they have a chemistry of the type: M'4/3M"2/3XTx. Due to the difference in atomic size between M' and M" they have ordering within the structure. This article is a nice overview and has some beautiful TEM images: Article i ‐MXenes for Energy Storage and Catalysis
Note, these structures only exist as M2XTx MXenes! For these, if you etch them with varying amounts of time, for some chemistries, you can remove M", leading to the ordered divacancy structure M4/3XTx.The other type of ordered structures are the out of plane (also called ordered double transition metal MXenes). These have a structure like Mo2Ti2C3Tx or Mo2TiC2Tx. In these, the Mo is on the outer layer, and Ti is on the inner layer. There is little intermixing between the two. This article has some nice TEM images that show this: Article Two-Dimensional, Ordered, Double Transition Metals Carbides (MXenes)
This article shows the different types of MXenes (and MAX) to help you understand: Article Characterization of MXenes at Every Step, from Their Precurs...
We provide a much more thorough description and list all of the known chemistries of those types.
MXenes have a general formula of M_n+1X_nTx, where M is a transition metal, X is carbon, nitrogen, or oxygen, T represents a surface termination group such as -OH or -F, and n is typically 1, 2, or 3.
The MXene structure is classified into three categories based on the type and number of transition metals present in the structure. These three categories are: mono-transition metal (MTM), double-transition metal (DTM), and divacancy transition metal (DVTM) MXenes.
DTM MXenes have two different transition metal elements in their structure, while DVTM MXenes contain a divacancy (missing two atoms) in the transition metal layer. Therefore, DVTM MXenes are distinct from DTM MXenes and not considered a subtype of DTM MXenes.
Regarding the terms "in-order" and "out-order" MXene, these terms refer to the stacking order of the MXene layers. In "in-order" MXenes, the transition metal layers are stacked in an ABAB... sequence, while in "out-order" MXenes, the stacking sequence is ABAC... or ABCA.... The terms "in-order" and "out-order" do not relate to the type of MXene structure, and both "in-order" and "out-order" MXenes can belong to any of the three categories of MXene structures (MTM, DTM, or DVTM).
Dear Shruti Deshpande
Please find the attached open-access paper. In the introduction, the MXenes structures are presented:
Article Progress and Insights in the Application of MXenes as New 2D...
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