Comment on ‘Multilayered Stable 2D Nano-Sheets of Ti2NTx MXene: Synthesis, Characterization, and Anticancer Activity’

<p>A
recent article entitled “Multilayered stable 2D nano-sheets of Ti<sub>2</sub>NT<sub>x</sub>
MXene: synthesis, characterization, and anticancer activity” published in
this journal, claimed that two-dimensional Ti<sub>2</sub>NT<sub>x</sub> MXene could be
synthesized by selectively etching Ti<sub>2</sub>AlN in concentrated
hydrofluoric acid at room temperature.
However, the X-ray diffraction pattern of Ti<sub>2</sub>NT<sub>x</sub> MXene reported in that paper is
completely different with those of other MXenes. In this comment, it is argued
that the samples synthesized in that paper were NOT Ti<sub>2</sub>NT<sub>x</sub>
MXene at all. Although carbide MXenes can be made by selectively etching A
layers from MAX phase, it is very difficult or impossible to make nitride
MXenes (Ti<sub>2</sub>NT<sub>x</sub>) by the same way.</p>


Background
The recent paper by Szuplewska et al. published in Journal of Nanobiotechnology [1] is on the synthesis of Ti 2 NT x MXene. MXenes are novel two-dimensional (2D) materials, which are prepared by selectively etching away A-site element from MAX (M n+1 AX n ) phase with hydrofluoric acid (HF) or fluoride salts solutions [2,3], where M is an early transition metal, A usually belongs to groups A and VA, X is carbon and/or nitrogen. In general, MXenes are made from carbide MAX phases (e.g. Ti 3 AlC 2 ) [4] or carbonitride MAX phases (e.g. Ti 3 AlCN) [5]. It is very difficult to make MXene from nitride MAX. Ti 2 AlN is the most common nitride MAX phase. If Ti 2 AlN can be selectively etching, Ti 2 NT x MXene can be made and it is predicted to have excellent properties. However, there is only one report on the experimental synthesis of Ti 2 NT x MXene by Soundiraraju et al. at 2017 [6]. In that report [6], it was claimed that Ti 2 NT x MXene was successfully made from Ti 2 AlN by KF/HCl etching. However, as the authors know, there are no reports to repeat that work.
The commented paper published in 2019 [1] is the second paper to claim the successful synthesis of Ti 2 NT x MXene from Ti 2 AlN. The etching solution is HF. However, the X-ray diffraction (XRD) patterns of Ti 2 NT x MXene in the paper are completely different with that in the first paper in 2017 [6]. Although the experiment in 2017 paper [6] has not been repeated, the XRD pattern of Ti 2 NT x MXene in that paper was similar with those of other MXenes (Ti 3 C 2 T x [7 ] or Ti 2 CT x [8 ]). The feature of MXenes' XRD patterns is that there is a very strong (002) peak and other peaks are very weak. However, the XRD pattern in the commented paper [1] is completely different with that in Ref. [6] and does not have the feature of MXenes' patterns. In addition, it was acknowledged in the paper [1] that the structure of Ti 2 NT x MXene was far from ideal. Moreover, in a recent work [9], it is theoretically indicated that Ti 2 NT x MXene cannot be successfully synthesized in HF solution from Ti 2 AlN, due to the fact that single layered nitride MXenes possess poor stability in the solution. Thus, we think that Ti 2 NT x MXene was not successfully synthesized in the commented paper.

Experiment and discussion
In order to prove this thought, the following experiment was carried out. 2 g Ti 2 AlN (< 25 μm, lab made) was immersed in two etching solutions with magnetic stirring. One solution was 20 ml hydrofluoric acid (HF, ≥ 40.0 wt.%; Yantai Far Eastern Fine Chemical Company, China). The other was 2 g KF (>99 wt.%; Aladdin, China) + 40 ml 6 M HCl (36 ~ 38 wt.%; Yantai Shuangshuang Chemical Company, China). The suspension after etching was further ultrasonically treated at room temperature for 1 h. Thereafter etched powders were separated from the suspensions by centrifuge. The powders were dried under vacuum at 60 °C for 8 h.
The XRD patterns are shown in Figure 1. Figure 1(a) is the XRD pattern of Ti 3 C 2 MXene in Reference [4]. Figure 1 Figure 2(I) in that paper). Thus those etched samples cannot be Ti 2 NT x MXene. The XRD patterns of those samples in Figure 1(b-c) agree well with the pattern of commercial TiN in Figure 1(d). All the three patterns can be perfectly indexed by PDF card No. 65-0715 in Figure 1(e). Therefore, the etched samples in this paper and those in the commented paper [1] are TiN powders with cubic crystal structure. They are NOT Ti 2 NT x MXene.

Conclusion
In summary, it is very difficult or impossible to make Ti 2 NT x MXene from Ti 2 AlN by selective etching of HF solution or KF + HCl solution. The obtained products in the commented paper was cubic TiN compounds rather than the claimed Ti 2 NT x with 2D structure.