Nanolaminated MAX Phases and Maxenes.

[NNadir]

The paper I'll discuss in this post is this one: Element Replacement Approach by Reaction with Lewis Acidic Molten Salts to Synthesize Nanolaminated MAX Phases and MXenes (Huang et al, J. Am. Chem. Soc. 2019, 141, 11, 4730-4737)

This morning, in this space, I wrote a post about the phase diagram of supercritical sodium chloride, a surrogate for seawater, arguing that heating seawater to a supercritical state might help to address some important environmental issues, including but not limited to climate change. That post is here:

The Phase Diagram of Supercritical Water + Sodium Chloride

After writing it, I poked around in the literature a bit, and came across the paper to which this post points.

The MAX phases and Mxenes, materials technology brought to prominence by the Egyptian-American scientist Michel W. Barsoum - nobody asks me but I feel he should be a candidate for a Nobel Prize - are remarkable materials that kind of fit a "best of" category between ceramics and metals.

The paper cited above takes these materials to a new level that inspires some thinking about energy technologies that might utilize them.


Here is a graphic cartoon introducing the paper:



From the introduction:

The family of nanolaminates called MAX phases and their two-dimensional (2D) derivative MXenes are attracting significant attention owing to their unparalleled properties.(1−7) The MAX phases have the formula Mn+1AXn (n = 1–3), where M is an early transition metal, A is an element traditionally from groups 13–16, and X is carbon or nitrogen. The unit cell of MAX phases is composed of M6X octahedral (e.g., Ti6C) interleaved with layers of A elements (e.g., Al). When etching the A-site atoms by HF or other acids, the retained Mn+1Xn sheets form 2D sheets, called MXenes. Theses 2D derivatives show great promise for applications such as battery electrodes, supercapacitors, electromagnetic absorbing and shielding coatings, catalysts, and carbon capture.(8−15)
So far, about 80 ternary MAX phases have been experimentally synthesized, with more continuously being studied, often guided by theory.(16,17) However, MAX phases with the A-site elements of late transition metal (e.g., Fe, Ni, Zn, and Pt), which are expected to exhibit diverse functional properties (e.g., magnetism and catalysis), are difficult to synthesize. For these late transition metals, their M-A intermetallics are usually more stable than the corresponding MAX phases at high synthesis temperatures, which means that the target MAX phases can hardly be achieved by a thermodynamic equilibrium process such as hot pressing (HP) and spark plasma sintering (SPS).

In 2017, Fashandi et al. synthesized MAX phases with noble-metal elements in the A site, obtained through a replacement reaction.(18,19) The replacement reaction was achieved by the replacement of Si by Au in the A layer of Ti3SiC2 at high annealing temperature with a thermodynamic driving force for the separation of Au and Si at moderate temperature, as determined from the Au–Si binary phase diagram. The formation of Ti3AuC2 is driven by an A-layer diffusion process. Its formation is preferred over competing phases (e.g., Au–Ti alloys), and the MAX phase can be obtained at a moderate temperature. Similarly, Yang et al. also synthesized Ti3SnC2 by a replacement reaction between the Al atom in Ti3AlC2 and SnO2,(20) although Ti3SnC2 can also be synthesized by a hot isostatic pressing (HIP) route.(21) Their work implies the feasibility of synthesizing novel MAX phases through replacement reactions.

It is worth noting that the synthesis of MAX phases by the A-site element exchange approach is similar to the preparation of MXenes by an A-site element etching process. Both are top-down routes that make modification of the A atom layer of pre-existing structure of the MAX phase, which involve the extraction of A-site atoms and the intercalation of new species (e.g., metallic atoms or functional terminals) at a particular lattice position. On the basis of this idea, we introduce here a general approach to synthesizing a series of novel nanolaminated MAX phases and MXenes based on the element exchange approach in the A-layer of the traditional MAX phase. The late transition-metal halides (e.g., ZnCl2 in this study) are so-called Lewis acids in their molten state.(22−25) These molten salts can produce strong electron-accepting ligands, which can thermodynamically react with the A element in the MAX phases. Simultaneously, certain types of atoms or ions can diffuse into the two-dimensional atomic plane and bond with the unsaturated Mn+1Xn sheet to form corresponding MAX phases or MXenes. The flexible selection of salt constituents can provide sufficient room to control the reaction temperature and the type of intercalated ions. In the present work, a variety of novel MAX phases (Ti3ZnC2, Ti2ZnC, Ti2ZnN, and V2ZnC) and MXenes (Ti3C2Cl2 and Ti2CCl2) were synthesized by elemental replacement in the A atomic plane of traditional MAX phases in ZnCl2 molten salts. The results indicate a general and controllable approach to synthesizing novel nanolaminated MAX phases and the derivation of halide-group-terminated MXenes from its respective parent MAX phase.

Some results from the paper:

Zn-MAX Phases

Ti3ZnC2 was prepared by using the starting materials of Ti3AlC2 and ZnCl2 with a mole ratio of 1:1.5. Figure 1a shows the XRD patterns of initial phase Ti3AlC2 and final product Ti3ZnC2. Compared to Ti3AlC2, the XRD peaks of Ti3ZnC2 (e.g., (103), (104), (105), and (000l) peaks) are shifted toward lower angles, indicating a larger lattice constant caused by the replacement of Al atoms by Zn atoms. Note that the relative intensity of (0004), (0006) peaks increased while that of (0002) peaks decreased. This is caused by the change in structure factor by the replacement of the A atoms. According to a Rietveld refinement of the XRD pattern (Figure S2), the determined a and c lattice parameter of Ti3ZnC2 are 3.0937 and 18.7206 Å, which are larger than those of Ti3AlC2 (a = 3.080 Å, c = 18.415 Å).(3)

Figure 1:



The caption:

Figure 1. Characterization of Ti3ZnC2. (a) XRD patterns of Ti3AlC2 and Ti3ZnC2. (b) SEM image of Ti3AlC2. (c) SEM image of Ti3ZnC2. (d, e) High-resolution (HR)-STEM image of Ti3ZnC2, showing the atomic positions from different orientations. (f) HR-STEM and the corresponding EDS map of Ti3ZnC2.

More text:

Figure 2 shows the phase identification of products derived from M2AX precursors. (Ti2AlC, T2AlN, and V2AlC were studied here.) The XRD patterns in Figure 2a exhibit a similar variation to that of the M3AX2 system, with (103), (106), and (110) peaks shifted toward lower angles and the relative intensity of the (000l) peaks changed. The STEM images (Figure 2b,c) and the EDS analysis (Figure S4) further confirmed the formation of the corresponding Zn-M2AX (T2ZnC, Ti2ZnN, and V2ZnC) after the reactions.

Figure 2:



The caption:

Figure 2. Characterization of Ti2ZnC, Ti2ZnN, and V2ZnC. (a–c) XRD patterns showing the three MAX phases with their respective precursors, where the arrows indicate the difference. (d–f) HR-STEM images of Ti2ZnC, Ti2ZnN, and V2ZnC, showing the atomic positions in different orientations.

And now for something really, really, really, really cool, the chloroMAX phases:

Cl-MXenes

A mixture of Ti3C2Cl2 MXene sheets and Zn spheres was obtained by using the starting materials of Ti3AlC2 and ZnCl2 with a mole ratio of 1:6. The SEM image (Figure 3a) and TEM images (Figure S5) of Ti3C2Cl2 show exfoliation along the basal planes. The corresponding EDS analysis (Figure S6) indicates that the elemental composition of MXene is Ti/C/Cl = 43.2:21.5:25.3 in atomic ratio, with small amounts of Zn (0.7 atom %), Al (2.9 atom %), and O(6.3 atom %). The presence of oxygen is reasonable because of prevailing O-containing compounds such as Al(OH)3, which is the hydrolysis product of AlCl3. Note that our theoretical calculation results indicate that the Cl terminations can strongly bond to the MXene surfaces but are not competitive with O-containing terminals.(40) Thus, a small part of the Cl terminations might be replaced by O-containing terminals during processes such as water washing, which could also contribute to the oxygen element detected on the surface. In addition, large Zn spheres can also be observed in the product, which can be easily distinguished from the MXenes (Figure S7)...

...A Ti 2p X-ray photoelectron spectrum of the as-reacted sample was shown in Figure 3d. The peak at 454.4 and 455.7 eV are assigned to the Ti–C (I) (2p3/2) and Ti–C (II) (2p3/2) bond.(41,42) The peak at 458.1 eV, attributing to a high valence Ti compound, is assigned to the Ti–Cl (2p3/2) bond.(43,44) Besides, the peaks at 460.3 eV, 461.8 and 464.1 eV are assigned to the Ti–C (I) (2p1/2), Ti–C (II) (2p1/2), and Ti–Cl (2p1/2) bonds, respectively...

Figure 3:



The caption:

Figure 3. Characterization of Ti3C2Cl2. (a) XRD patterns from the as-reacted product and HCl-treated product. (b) SEM image showing Ti3C2Cl2 in the as-reacted sample. (c) HR-STEM image showing the atomic positions of Ti3C2Cl2. (d) Ti 2p XPS analysis of the as-reacted product. (e) Cl 2p XPS analysis of the as-reacted product. (f) Band structure of Ti3C2Cl2.

The synthesis of these compounds is fairly straight forward, and involves heating the max phases in a ZnCl2 molten salt for zinc substitution, mixed KCl and NaCl molten salts for the chlorinated species.

The reaction progress was monitored by Energy Dispersive Spectroscopy (EDS) and X-ray diffraction (XRD).

Figure 4:

Figure 4. Phase evolution. (a) XRD patterns of the product of the Ti3AlC2/ZnCl2 = 1:6 system with different reaction times, showing the phase evolution from Ti3AlC2 toTi3ZnC2 to Ti3C2Cl2. (b) EDS mapping analysis of the 1.5 h sample showing the Ti3ZnC2@Ti3C2Cl2 core–shell structure.

From the conclusion:

A series of novel MAX phases (Ti3ZnC2, Ti2ZnC, Ti2ZnN, and V2ZnC) and Cl-terminated MXenes (Ti3C2Cl2 and Ti2CCl2) were synthesized by a replacement reaction, where the A element in traditional MAX phase precursors is replaced with Zn from the Zn2+ cation in molten ZnCl2.
The formation of the Zn-MAX phases was achieved by a replacement reaction between Zn2+ and Al and subsequently the occupancy of Zn atoms in the A sites of the MAX phase. The formation of Zn-MAX phases indicates that such an exchange mechanism between traditional Al-MAX phases and the late transition metal halides might be a general approach for synthesizing some other unexplored MAX phases with functional A-site elements (such as magnetic element Fe). Late transition metal halides (e.g., ZnCl2), which have relatively low melting points and exhibit strong Lewis acidity in their molten state, seem to be ideal candidates for the replacement reaction. The acidic environment provided by the molten salts facilitates the extraction of the Al atoms from the A-atom plane in the MAX phase at a moderate temperature. The generation of the volatile Al halides in turn provides the driving force for the outward diffusion of the Al atom. Meanwhile, the liquid environment also facilitates the inward diffusion of replacement atoms, which finally promotes a thorough replacement reaction...

I have long wondered about the possibility of extending the MAX phase type structures across the periodic table beyond the early d-transition elements.

This is not my professional interest, but it is very, very, very, very cool, if esoteric.

I hope you're having a nice afternoon.