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Thu Jun 10, 2021, 08:44 PM

The world of two-dimensional carbides and nitrides (MXenes)

There's a nice review article on these amazing structures, first derived from the ternary MAX phases, first studied in great detail by (but not discovered by) Michel Barsoum, who - not that anyone cares what I think - I think should be a candidate for the Nobel Prize.

Dr. Barsoum is an Arab American scientist at Drexel University. My son had an opportunity to sit for a time with him while attending an "accepted students" meeting at Drexel, but took a better financial offer elsewhere. I was slightly disappointed that Drexel hadn't made a competitive offer, because well, Dr. Barsoum...

I'd been studying the MAX phase literature for some time, owing to my strong interest in refractory materials and titanium chemistry. (The most famous MAX phase is Ti3SiC2. )

The MAX phases are materials that combine the best properties of metals, electrical conductivity, machinability and fracture resistant strength, with the best properties of ceramics, high melting points, resistance to corrosion and resistance to deformation. They generally form in layered structures, or if not layered, in highly structured arrays.

The MAXenes are made by special dissolution properties acting on MAX phases, for example, the dissolution of Silicon layers using HF.

The review article here: The world of two-dimensional carbides and nitrides (MXenes). (Armin VahidMohammadi, Johanna Rosen, Yury Gogots, Science 372, eabf1581 (2021))

Two of the authors are from Drexel, and one from Sweden.

It's a review article, and I cannot cover much of it, but if you happen to get access to Science, it is a very interesting topic.

I can share the introduction to the review and a few graphics to get a feel for the topic.

A brief excerpt of the introduction:

MXenes are a large family of two-dimensional (2D) metal carbides and nitrides having a structure consisting of two or more layers of transition metal (M) atoms packed into a honeycomb-like 2D lattice that are intervened by carbon and/or nitrogen layers (X atoms) occupying the octahedral sites between the adjacent transition metal layers (1, 2). MXenes are produced through a top-down synthesis approach, where typically A-layer atoms (e.g., Al, Si, Ga) are selectively removed from the structure of MAX phases, a group of layered, hexagonal-structure ternary carbides and nitrides (3), leaving behind loosely stacked MX layers [called “MXene” to emphasize their 2D nature (2)], which can be further separated into single-layer flakes (Fig. 1).

Ti3C2Tx was made by selective etching of monoatomic Al layers from the Ti3AlC2 MAX phase precursor in hydrofluoric (HF) acid (1). High metallic conductivity, hydrophilicity, and the ability to intercalate cations and store charge, demonstrated by Ti3C2Tx (4, 5) and other MXenes (6, 7), initially led to interest in exploring MXenes for energy storage applications. The year 2017 was the beginning of the MXenes “gold rush.” Since then, the world of 2D carbides and nitrides has been growing at an unprecedented rate. There are currently more than 30 different experimentally made stoichiometric MXenes and more than a hundred (not considering surface terminations) theoretically predicted MXenes (8–10) with distinct electronic, physical, and (electro)chemical properties. In addition, solid solutions on M and X sides are possible, and the possibility of having multiple single (O, Cl, F, S, etc.) or mixed (O/OH/F) surface terminations makes MXenes a large and diverse family of 2D materials.

The variety of MXene structures and compositions (Fig. 1) makes it necessary to define a terminology for MXenes. The general formula of MXenes is Mn+1XnTx, where M represents the transition metal site, X represents carbon or nitrogen sites, n can vary from 1 to 4, and Tx (where x is variable) indicates surface terminations on the surface of the outer transition metal layers (8, 11). As an example, the chemical formula of a titanium carbide MXene with two layers of transition metal (n = 1) and random terminations would be Ti2CTx, and a completely oxygen- or chlorine-terminated Ti2CTx can be written as Ti2CO2 or Ti2CCl2, respectively. If there are two randomly distributed transition metals occupying M sites in the MXene structure forming a solid solution, the formula will be written as (M′,M′?n+1XnTx, where M′ and M′′ are two different metals [e.g., (Ti,V)2CTx)]...

A few figures:

The caption:

Fig. 1 Schematic illustration of the MXene structures.

2D MXenes have a general formula of Mn+1XnTx, where M is an early transition metal, X is carbon and/or nitrogen, and Tx represents surface terminations of the outer metal layers. The n value in the formula can vary from 1 to 4, depending on the number of transition metal layers (and carbon and/or nitrogen layers) present in the structure of MXenes, for example, Ti2CTx (n = 1), Ti3C2Tx (n = 2), Nb4C3Tx (n = 3), and (Mo,V)5C4Tx (n = 4). In contrast to all previously known MXenes, the recently discovered Mo4VC4Tx solid solution MXene with five M layers shows twinning in the M layers (146). The M sites of MXenes can be occupied by one or more transition metal atoms, forming solid solutions or ordered structures. The ordered double transition metal MXenes exist as in-plane ordered structures [i-MXenes, e.g., (Mo2/3Y1/3)2CTx]; in-plane vacancy structures (e.g., W2/3CTx); and out-of-plane ordered structures (o-MXenes), where either one layer of M′′ transition metal is sandwiched between two layers of M′ transition metal (e.g., Mo2TiC2Tx) or two layers of M′′ transition metals are sandwiched between two layers of M′ transition metals (e.g., Mo2Ti2C3Tx). Other arrangements, such as one or three layers of M′′ sandwiched between the layers of M′ (bottom row) in the M5X4 structure, may be possible. Faded images at the bottom of the figure represent predicted structures such as high-entropy MXenes and higher-order single M or o-MXenes that have yet to be experimentally verified.

The caption:

Fig. 2 Electronic, optical, and mechanical properties of MXenes.
(A) Schematic illustration of different compositional and structural factors determining electronic and optical properties of MXenes. (B) Total DOS for Mo2TiC2O2 and Mo2Ti2C3O2, showing the effect of MXene structure (40). (C) DOS of Ti3C2, Ti3C2O2, Ti3C2(OH)2, and Ti3C2F2, showing the effect of surface chemistry on electronic properties of MXenes (44). (D) Dependence of the work function of MXenes on their surface chemistry (45). (E) The color of colloidal solutions of various MXenes and their corresponding freestanding films (54). (F) Digital photographs of three M′2-yM″yCTx solid solution systems, showing the change in optical properties and color of freestanding MXene films due to a gradual change in the stoichiometry (55). (G) UV-vis-NIR optical extinction properties of aqueous dispersions of various 2D transition metal carbides (54). (H) UV-vis-NIR transmittance spectra from 300 to 2500 nm for MXene thin films (54). (I) Tensile stress versus strain curves of Ti3C2Tx films with different thickness produced by vacuum-assisted filtration and blade coating (60). (J) Force-deflection curves of a bilayer Ti3C2Tx flake at different loads. The lower inset is a detailed view of the same curves showing the center of origin. The top inset shows an AFM image of a punctured flake with no sign of catastrophic rupture (61). (K) Comparison of the effective Young’s moduli of single-layer Ti3C2Tx and Nb4C3Tx with other 2D materials tested in similar nanoindentation experiments (62). [(B) to (K) adapted with permission from (40, 44, 45, 54, 55, 60–62)]

The caption:

Fig. 3 Synthesis and processing of MXenes.

(A) Schematic illustration of two approaches to produce MXenes by removal of A layers from MAX phases and related layered compounds. In the first approach, the MAX phase is selectively etched in fluoride ion–containing acids. By this method, multilayered MXene particles or in situ delaminated 2D flakes (using the MILD method) can be obtained. In the second approach, the MAX phase is selectively etched in molten salts. The product is usually multilayered MXene particles, which can then be delaminated through intercalation. (B) Scanning electron microscope (SEM) image of a hexagonal-shape Ti3AlC2 MAX phase crystal (52). (C) SEM image of a Ti3C2Tx MXene particle, derived from Ti3SiC2 by selective etching of Si layers in molten salt (36). (D) Top-view SEM image of a delaminated Ti3C2Tx flake (52). (E) STEM image of a M3AX2 MAX phase (Ti3AlC2) particle. (F) The corresponding STEM image of an ml-M3X2Tx MXene particle (Ti3C2Tx). (G) Atomically resolved plane-view STEM image of single-layer Ti3C2Tx (28). (H to J) Digital photographs of ~1 L of delaminated Ti3C2Tx solution (166), highly concentrated Ti3C2Tx ink (167), multilayered Ti3C2Tx MXene particles (166), a Ti3C2Tx film prepared by vacuum-assisted filtration of a colloidal MXene solution (168), and large-area, mechanically robust Ti3C2Tx film produced by blade coating (60). [(E) and (F) courtesy of P. O. Å. Persson; (B) to (D) and (G) to (J) adapted with permission from (14, 28, 36, 52, 60, 166, 167)]

I hope to find some time to spend with this article. These are ground breaking materials.

Have a nice day tomorrow.

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Reply The world of two-dimensional carbides and nitrides (MXenes) (Original post)
NNadir Jun 10 OP
eppur_se_muova Jun 11 #1
NNadir Jun 12 #2

Response to NNadir (Original post)

Fri Jun 11, 2021, 10:43 PM

1. Imagine seeing something like IC technology in its first few years ....

... and imagining all the different directions it could go, and what an impact it could have. This has the same aura about it -- maybe not on such a broad scale, but big things are going to come of this.

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Response to eppur_se_muova (Reply #1)

Sat Jun 12, 2021, 08:37 AM

2. This is true; it does seem that the number of publications devoted to them is rising.

The most interesting application I've seen this far is as a lubricant, of all things, presumably stable at high temperatures. I can imagine many many applications as a lubricant, particularly in very high temperature nuclear energy settings where the goal is to increase exergy via heat networks.

The MAX phases themselves are extremely interesting. There are a quite a number of them. They can be made from early transition metals, group 13, 14, and 15 elements plus either carbon or nitrogen.

Here, from an open sourced paper, Processing of MAX phases: From synthesis to applications (Jesus Gonzalez-Julian, J. Amer. Ceram. Soc. 104, 2 (2021) 659-690) is a periodic table showing the elements from which they've been made:

I think the MAX Phases will change the world; I'm encouraging my son to not forget about them, even though they are not involved in his research now, they may be very important throughout his career, and it's good to have a head start.

I first turned him on to them when he was in high school.

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