Covalent surface modifications and superconductivity of two-dimensional metal carbide MXenes

[NNadir]

The paper I will discuss in this post is this one: Covalent surface modifications and superconductivity of two-dimensional metal carbide MXenes (Vladislav Kamysbayev, Alexander S. Filatov, Huicheng Hu, Xue Rui, Francisco Lagunas, Di Wang, Robert F. Klie, Dmitri V. Talapin, Science 21 Aug 2020: Vol. 369, Issue 6506, pp. 979-983)

My son decided that he wanted to go into materials science while still in high school, and as such, we visited the Materials Science Departments of the various universities during the touring process. I attended the majority of them, but my wife took my son to Drexel University where, I learned afterward, the great Egyptian-American scientist Michel Barsoum actually came to speak to the prospective students, this, ironically on the very day I had acquired access to his book, MAX Phases: Properties of Machinable Ternary Carbides and Nitrides



Of course, if I had attended, any effort on my part to have engaged Dr. Barsoum would have distracted from his mission, which was to convince promising students to come to Drexel where, according to my wife, he promised, if they worked hard, even freshman undergraduates could be invited to work in his lab.

Drexel made my son a decent offer but the university he ultimately attended made him a great offer, and anyway, my son really found the idea of attending a university located right in a major metropolitan area distasteful, which is why he refused to even look at NYU, Columbia or MIT, not that any of these universities would have made him an offer we could have afforded to accept, or for that matter, even admitted him. So he didn't go to Drexel, and he didn't get to work with Michel Barsoum, even though his father had been discussing the MAX phases with him for some time.

In my opinion, however, Dr. Barsoum is one of the most important scientists of our time. He did not discover the MAX Phases, but he recognized them for what they were, greatly expanded on the knowledge of their chemistry and properties, and as published with scientists all over the world on the subject.

The MAX phases (which Dr. Barsoum named) have many of the important features of ceramics, resistance to high temperatures, resistance to harsh chemicals, while possessing some of the important properties of metals, specifically, machinability, as they lack the brittle nature of ceramics. I came across them in connection with my interest in high temperature materials and chemical resistance given my interest in nuclear reactors as well as in thermochemical carbon dioxide and water splitting using them in order to make for a sustainable world, something that we are no closer to doing than when I was a child; in fact we are living in a less sustainable world than the one into which I was born. (History will not forgive my generation, nor should it.) In any case, structurally, MAX phases consist of layers of atoms in a fairly precise arrangement, and this, as Dr. Barsoum and others have taught the world, leaves them capable of offering new opportunities in materials science in many areas.

One area in which Dr. Barsoum has further pioneered the applicability of these materials is in their use in preparing "MAXenes" which are two dimensional layered materials having a single molecule thickness. Although the MAX phases are notable for their chemical resistance, there are some which do react with chemicals. The most famous MAX phase - there are many, but the most famous - is Ti3SiC2. If this phase is treated with hydrofluoric acid, the silicon in them can be dissolved, leaving a two dimensional series of layers of Titanium carbide. The invention of the FFC Cambridge process should make titanium metal readily available in the future at reasonable prices, and the properties of its carbides (and indeed, the already widely used nitride) are very, very, very, exciting.

Much of what is written today on the subject of two dimensional materials these days relates to graphene and graphene nitride. MAXenes open up a much larger segment of the periodic table to these types materials.

Modification to MAXene titanium carbides is the subject of the paper under discussion and it extends the elements of the period table to two dimensional materials to the halides.

From the introduction to the paper:

Two-dimensional (2D) transition-metal carbides and nitrides (MXenes) (1) have been actively studied for applications in supercapacitors (2), batteries (3), electromagnetic interference shielding (4), composites (5, 6), and catalysts (7). MXenes are typically synthesized from the corresponding MAX phases (Fig. 1A), where M stands for the transition metal (e.g., Ti, Nb, Mo, V, W, etc.) and X stands for C or N, by selectively etching the main group element A (e.g., Al, Ga, Si, etc.). The etching is usually performed in aqueous hydrofluoric (HF) solutions, rendering MXenes terminated with a mixture of F, O, and OH functional groups, commonly denoted as Tx. These functional groups can be chemically modified, unlike the surfaces of other 2D materials such as graphene and transition-metal dichalcogenides. Recent theoretical studies predict that selective terminations of MXenes with different surface groups can lead to remarkable properties, such as opening or closing bandgap (8), room-temperature electron mobility exceeding 104 cm2/V?s (9), widely tunable work functions (10), half-metallicity, and 2D ferromagnetism (11). Covalent functionalization of MXene surfaces is expected to uncover new directions for rational engineering of 2D functional materials

The surface of MXene sheets is defined during MAX phase etching. Electrochemical and hydrothermal methods have been recently applied for etching MAX phases without resorting to HF solutions, but the use of aqueous solutions introduces a mixture of Cl, O, and OH surface groups (12, 13). The etching of Ti3AlC2 MAX phase in molten ZnCl2 and several other Lewis acidic molten salts above 500°C results in Ti3C2Cl2 MXene with a pure Cl termination (14, 15). Because etching of MAX phases in molten salts eliminates unwanted oxidation and hydrolysis, we used a variation of this method for synthesis of Ti3C2Cl2, Ti2CCl2, and Nb2CCl2 MXenes in CdCl2 molten salt (figs. S1 to S5). Moreover, the use of Lewis acidic CdBr2 allowed us to extend the molten salt etching route beyond chlorides to prepare the first Br-terminated Ti3C2Br2 and Ti2CBr2 MXenes (Fig. 1, B and C, and figs. S6 and S7)...

Figure 1:



The caption:

Fig. 1 Surface reactions of MXenes in molten inorganic salts.
(A) Schematics for etching of MAX phases in Lewis acidic molten salts. (B) Atomic-resolution high-angle annular dark-field (HAADF) image of Ti3C2Br2 MXene sheets synthesized by etching Ti3AlC2 MAX phase in CdBr2 molten salt. The electron beam is parallel to the [21¯1¯0] zone axis. (C) Energy-dispersive x-ray elemental analysis (line scan) of Ti3C2Br2 MXene sheets. a.u., arbitrary units. HAADF images of (D) Ti3C2Te and (E) Ti3C2S MXenes obtained by substituting Br for Te and S surface groups, respectively. (F) HAADF image of Ti3C2⬜⬜2 MXene (⬜ stands for the vacancy) obtained by reductive elimination of Br surface groups.

Here is a excerpted brief discussion of the chemical processing of these phases:

The transition-metal atoms from the outer layers of MXene sheets (Ti, Mo, Nb, and V) form relatively weak M-Cl and M-Br bonds, in comparison to M-F and M-OH bonds typical for MXenes with Tx surface groups. This point can be demonstrated by the enthalpies of formation for TiBr4 (?617 kJ mol?1) and TiCl4 (?804 kJ mol?1) versus TiF4 (?1649 kJ mol?1), as well as by direct comparison of the bond energies (table S1). Strong Ti-F and Ti-O bonds make it difficult to perform any postsynthetic covalent surface modifications of MXenes (16). In contrast, Cl- and Br-terminated MXenes with labile surface bonding act as versatile synthons for further chemical transformations.

MXene surface exchange reactions typically require temperatures of 300° to 600°C, which are difficult to achieve using traditional solvents. We instead used molten alkali metal halides as solvents with unmatched high-temperature stability, high solubility of various ionic compounds, and wide electrochemical windows (17–19). For example, Ti3C2Br2 MXene (Fig. 1B) dispersed in CsBr-KBr-LiBr eutectic (melting point: 236°C) reacted with Li2Te and Li2S to form Ti3C2Te (Fig. 1D and figs. S8 to S10) and Ti3C2S (Fig. 1E and fig. S11) MXenes, respectively. The reactions of Ti3C2Cl2 and Ti3C2Br2 with Li2Se, Li2O, and NaNH2 yielded Ti3C2Se, Ti3C2O, and Ti3C2(NH) MXenes, respectively (figs. S12 to S16). The multilayers of Ti3C2Tn MXenes (T = Cl, S, NH) were further treated with n-butyl lithium (n-BuLi) resulting in Li+ intercalated sheets (fig. S17) with a negative surface charge (Fig. 2A and fig. S18)

A graphic on delamination of the MAXenes:



The caption:

Fig. 2 Delamination of multilayer Ti3C2Tn MXenes.

(A) Schematic of delamination process. (B) Photographs of stable colloidal solutions of Ti3C2Tn MXenes (T = Cl, S, NH) in NMF exhibiting Tyndall effect. (C) TEM image of Ti3C2Cl2 MXene flakes deposited from a colloidal solution. (Inset) Fast Fourier transform of the circled region, showing crystallinity and hexagonal symmetry of the individual flake. (D) XRD patterns of multilayer MXene and delaminated flakes in a film spin coated on a glass substrate.

There is considerable discussion in the paper of various means and results of characterization, including a discussion of the electrical properties of these materials.

The above examples show that the composition and structure of MXenes can be engineered with previously unattainable versatility. Chemical functionalization of MXene surfaces is expected to affect nearly every property of these materials, and we found that the surface groups defined the nature of electronic transport in Nb2CTn MXenes. Figure 4, A and B, shows temperature-dependent four-probe resistivity (? measured on cold-pressed pellets of Nb2CTn (T = ⬜, Cl, O, S, Se) MXenes (fig. S41), all synthesized by the procedures described above. Figure 4A also compares the conductivity of the parent Nb2AlC MAX phase with that of Nb2CCl2 MXene. Above 30 K, both MAX phase and MXene samples showed similar specific resistivity, which decreased when the sample was cooled. This temperature dependence is often associated with metallic conductivity. The ultraviolet photoelectron spectroscopy (UPS) confirmed nonzero density of electronic states at the Fermi energy EF (fig. S42), which is also consistent with a metallic state.

Figure 4:



The caption:

Fig. 4 Electronic transport and superconductivity in Nb2CTn MXenes.
(A) Temperature-dependent resistivity for the cold-pressed pellets of Nb2AlC MAX phase and Nb2CCl2 MXene. (Inset) Magnetic susceptibility (i.e., ratio of magnetization to magnetizing field strength) of Nb2CCl2 MXene as a function of temperature. FC and ZFC correspond to the field cooled and zero-field cooled measurements, respectively. emu, electromagnetic unit. (B) Temperature-dependent resistivity for the cold-pressed pellets of Nb2CTn MXenes. (Inset) Resistance as a function of temperature at different applied magnetic fields (0 to 8 T) for the cold-pressed pellets of Nb2CS2 MXene.

I never get too excited about applications of niobium, since niobium is a monoisotopic (A = 93) element that is subject to depletion of resources and which cannot be obtained from used nuclear fuel owing to the long half-life of its parent, Zr-93.

In any case, the authors continue:

However, when the Nb2CCl2 MXene was cooled below 30 K, the resistivity started increasing, possibly indicating the onset of localization. A sharp drop of resistivity by several orders of magnitude occurred at a critical temperature Tc ~ 6.0 K (Fig. 4A), which is reminiscent of a superconductive transition. The magnetic susceptibility measurements showed the development of a strong diamagnetism below 6.3 K that we interpreted as the Meissner effect (Fig. 4A). From the magnitude of zero-field cooled data at 1.8 K, we estimated the lower bound for the superconducting volume fraction of Nb2CCl2 MXene as ~35%. Consistent with superconductivity, the transition broadened, and Tc shifted to lower temperatures with the application of an external magnetic field (Fig. 4B and fig. S43). In contrast, the parent Nb2AlC MAX phase exhibited normal metal behavior down to the lowest measured temperature (1.8 K), which is consistent with a previously reported Tc ~ 0.44 K for Nb2AlC (28). For reference, Nb2CTx MXene with mixed O, OH, and F termination prepared by the traditional aqueous HF etching route shows two orders of magnitude higher resistivity and no superconductivity (fig. S44) (29).

In the conclusion the authors suggest a breakthrough in MAXene processing:

The MXene exchange reactions represent an exciting counterexample to the traditional perception of solids as entities that are difficult to postsynthetically modify. We showed that chemical bonds inside an extended MXene stack can be rationally designed in a way that is more typical for molecular compounds. Other MXene structures could be enabled by the combinations of etching and substitution reactions using Lewis acidic and Lewis basic molten salts, respectively.

It's a cool paper on what I regard as an important area in the future of materials science.

I trust you are having a pleasant weekend and enjoying the excitement over our outstanding virtual Democratic Party and are filled, as I am, with feelings of hope.