# In Situ X-ray Diffraction Study of MXene Synthesis by the Reaction of Ti3AlC2 with Molten Zinc and Tin Chlorides

Artem Iakunkov, Nicolas Boulanger, Bartosz Gurzeda, Gui Li, Christoph Hennig, Volodymyr Svitlyk, Mads Ry Vogel Jørgensen, Innokenty Kantor, Igor A. Baburin, Mahiar Max Hamedi, et al.

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To cite this version:

Artem Iakunkov, Nicolas Boulanger, Bartosz Gurzeda, Gui Li, Christoph Hennig, et al. In Situ X-ray Diffraction Study of MXene Synthesis by the Reaction of Ti3AlC2 with Molten Zinc and Tin Chlorides. Chemistry of Materials, 2025, 37 (3), pp.1132-1142. 10.1021/acs.chemmater.4c02989. http://dx.doi.org/10.1021/acs.chemmater.4c02989

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# In Situ X-ray Diffraction Study of MXene Synthesis by the Reaction of Ti₃AlC₂ with Molten Zinc and Tin Chlorides

Artem Iakunkov, Nicolas Boulanger, Bartosz Gurzeda, Gui Li, Christoph Hennig, Volodymyr Svitlyk, Mads Ry Vogel Jørgensen, Innokenty Kantor, Igor A. Baburin, Mahiar Max Hamedi, and Alexandr V. Talyzin*

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Cite This: *Chem. Mater.* 2025, 37, 1132–1142

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**ABSTRACT:** Using molten salts for etching aluminum (Al) away from the MAX phase for MXene synthesis is an attractive alternative method that allows one to avoid the use of toxic hydrofluoric acid (HF) solutions. However, the mechanism of the MAX phase reaction with molten salts remains to date unclear due to the lack of in situ data. Here, we present a detailed in situ time-resolved synchrotron radiation X-ray diffraction study of the MAX phase annealing in molten ZnCl₂ and SnCl₂. The reaction of salts with the MAX phase is found to occur in two stages. The initial period of annealing results in the delamination of two-dimensional (2D) Ti₃C₂ layers, vigorous evolution of AlCl₃ bubbles, and dissolution of Zn in a ZnCl₂ melt. The chlorine-terminated Ti₃C₂ sheets formed in the delaminated state are restacked into a relatively well-ordered MXene structure (P6₃/mmc, *a* = 3.071 Å and *c* = 18.577 Å) during the prolonged annealing in molten salts. Surprisingly, the data recorded directly in molten salts at temperatures up to 873 K demonstrate that Ti₃C₂Clₓ MXene shows no swelling in both liquid ZnCl₂ and SnCl₂. The structure of MXene studied directly in the molten salts is found to be the same as in ex situ experiments performed after cooling and water washing under ambient conditions. The absence of the “pristine” melt-swollen phase indicates a rather different mechanism of MXene formation compared to HF-based solution methods. Formation of MXene by gradually removing Al from the MAX phase starting at the edges of flakes and propagating into the deeper parts of interlayers is not possible, since the molten salt is not capable of penetrating between Cl-terminated Ti₃C₂ layers.

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## INTRODUCTION

MXenes are materials composed of two-dimensional (2D) sheets of transition metal carbides.¹,² Strong interest in MXenes over the past decade has been motivated by the broad range of proposed applications, including energy storage,³,⁴ water purification,⁵ electromagnetic interference shielding,⁶ biomedical applications,⁷ wear-resistant coatings,⁸ and electronics.⁹

MXenes are synthesized by selectively etching “A” layers of metal atoms from their layered precursors (MAX phases) while retaining the MX layers with a two-dimensional structure.² The most studied MXene so far is Ti₃C₂Tₓ produced by etching aluminum (Al) away from the Ti₃AlC₂ structure.¹⁰,¹¹ Ti₃C₂Tₓ can be delaminated to produce true 2D sheets by dispersing it in suitable polar solvents, e.g., dimethyl sulfoxide (DMSO).¹¹,¹² The 2D Ti₃C₂ sheets comprising Ti-MXene are terminated by several kinds of functional groups, including fluorine, oxygen, and hydroxyls, denoted “Tₓ”, but the exact type of termination is still somewhat uncertain and depends on synthesis procedures.¹³,¹⁴

The most common MXene synthesis route is wet chemical etching using hydrofluoric acid (HF)¹⁰ and, more recently, the use of strong acids together with Li salts such as “HF + LiCl”¹³,¹⁵ or “LiF + HCl”.¹⁶ In the latter case, the etching

agent is again HF. Our recent in situ X-ray diffraction (XRD) experiments demonstrated that “pristine” MXene studied prior to water washing has a structure intercalated with one layer of HF solution.¹⁷ The etching of Al is then limited to one-layer thickness “channels” provided by the replacement of Al with HF molecules. Conventional MXene is produced by water washing, which needs to be considered as the last step of chemical synthesis. The reaction of pristine MXene with water removes HF intercalated between MXene layers, possibly also adding some oxygen groups to the surface of MXene sheets.¹⁷

Using toxic HF for the synthesis of MXenes is environmentally undesirable and is a serious hindrance to mass production. Therefore, alternative methods not involving HF for the synthesis of MXenes are of great importance for the wider implementation of these materials in realistic applications.

Received: October 28, 2024

Revised: January 10, 2025

Accepted: January 13, 2025

Published: January 21, 2025

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One of the most attractive alternatives to HF synthesis routes developed very recently is based on etching performed in molten salts serving as a Lewis acid.<sup>18-20</sup> The Lewis Acid Molten Salt Etching (LAMS) is fluoride-free but requires using relatively high temperatures for etching the Al layer from the MAX phase.<sup>18</sup> Zinc, tin, copper, and some other metal chlorides have been successfully used as Lewis acids for the preparation of Ti<sub>3</sub>C<sub>2</sub>T<sub>x</sub><sup>20-24</sup>.

Unlike the MXene sheets prepared using wet chemical methods, which are believed to be terminated (at least partly) by fluorine- and oxygen-containing functional groups, using molten chloride salts suggests that MXene sheets prepared by the LAMS method must be terminated by chlorine.<sup>20,24,25</sup> The experimental report supported by theoretical simulations suggests the composition of molten salt MXene as Ti<sub>3</sub>C<sub>2</sub>Cl<sub>2</sub>.<sup>25</sup>

Therefore, it is expected that some properties of MXenes prepared by these routes (e.g., stability in water) can be significantly different. In particular, several studies report that Ti<sub>3</sub>C<sub>2</sub>Cl<sub>x</sub> prepared by the LAMS method is difficult to disperse in aqueous solutions.<sup>26,27</sup> Therefore, modification of the surface chemistry of MXenes prepared by the LAMS method is also rather actively expanding the research field.<sup>28,29</sup>

The exact mechanism of MXene synthesis by the reaction with molten salts is also still under debate. It is suggested that the etching of Al from the MAX phase occurs by the reaction with, e.g., molten ZnCl<sub>2</sub> and the formation of gaseous AlCl<sub>3</sub> and metallic Zn. The Ti<sub>3</sub>C<sub>2</sub> layers formed after the removal of Al are terminated by chlorine, forming Ti<sub>3</sub>C<sub>2</sub>Cl<sub>x</sub>.<sup>20</sup> A relatively large excess of molten salt (mole ratio 6:1) is required for successful MXene synthesis from the MAX phase using the LAMS method. The existence of Zn-substituted MAX phase Ti<sub>3</sub>ZnC<sub>2</sub> was also reported for experiments with a relatively small amount of molten ZnCl<sub>2</sub> added to the MAX phase (mole ratio of 1.5:1).<sup>20</sup> However, the formation of the Ti<sub>3</sub>ZnC<sub>2</sub> phase is completely avoided in experiments with a large excess of molten salt.<sup>26</sup>

It is important to note that most studies of MXenes produced by the LAMS method are limited to ex situ analysis of materials after cooling the reaction mixture and extensive washing to eliminate other reaction products.<sup>18</sup> High temperatures of LAMS synthesis (600–750 °C) and the need to record data in liquid molten salts complicate in situ studies of the reaction mechanism, making it the perfect subject for synchrotron radiation XRD studies.

Only one in situ structural study of MXene synthesis by the LAMS method is so far available. This study reports XRD data for a relatively exotic Nb-MAX phase synthesized using the Nb<sub>2</sub>GaC precursor.<sup>30</sup> Surprisingly, so far, no in situ studies are available for the synthesis of the most common Ti<sub>3</sub>C<sub>2</sub>T<sub>x</sub> MXene phase using the Ti<sub>3</sub>AlC<sub>2</sub> MAX phase as a precursor. Considering that –OH-terminated Ti-MXene shows easy swelling in water<sup>31</sup> and aqueous HF solution,<sup>17</sup> we anticipated that Cl-terminated Ti-MXene would be exhibiting swelling in molten chloride salts. Surprisingly, this suggestion was not confirmed.

Here, we report the in situ synchrotron radiation XRD study of Ti<sub>3</sub>C<sub>2</sub>Cl<sub>x</sub> synthesis by the reaction of the Ti<sub>3</sub>AlC<sub>2</sub> MAX phase with molten ZnCl<sub>2</sub> and SnCl<sub>2</sub> at elevated temperature conditions. Complete transformation of the MAX phase into MXene was achieved in molten ZnCl<sub>2</sub>, and a detailed analysis of the sample materials as a function of annealing time is presented. The reaction proceeds with the formation of delaminated Ti<sub>3</sub>C<sub>2</sub> layers at the initial stage of annealing. It

is suggested that the removal of Al from the MAX structure, the termination of 2D titanium carbide layers with chlorine, and formation of elemental Zn dissolved in ZnCl<sub>2</sub> occur in the delaminated phase, which is an intermediate between MAX and MXene. The Ti<sub>3</sub>C<sub>2</sub>Cl<sub>x</sub> phase is found to form after prolonged annealing suggestively by restacking 2D layers of chlorine-terminated Ti<sub>3</sub>C<sub>2</sub> formed initially in the delaminated phase. The MXene formed in the molten salts (both ZnCl<sub>2</sub> and SnCl<sub>2</sub>) has the same structure and shows no swelling in the molten salts. The surprising absence of melt-swollen “pristine MXene” indicates a reaction mechanism that is rather different compared to HF-based aqueous solution methods.

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## EXPERIMENTAL SECTION

**Materials and Synthesis.** ZnCl<sub>2</sub> and SnCl<sub>2</sub> (reagent grade, 98%) were purchased from Sigma-Aldrich and used without additional purification. MAX phase Ti<sub>3</sub>AlC<sub>2</sub> precursor was purchased from Carbon-Ukraine.

For the synthesis of bulk samples, a powder mixture of Ti<sub>3</sub>AlC<sub>2</sub> and ZnCl<sub>2</sub> in a 1:6 molar ratio was loaded into ~7 mm diameter borosilicate glass tubes under nitrogen, and the tubes were sealed under dynamic vacuum (~10<sup>-5</sup> bar). The tubes with the sample were then heated in a standard tubular furnace with a rate of 8 K/min up to 853 K. After annealing at 853 K (1 min for one sample and 3 h for the other), the samples were cooled by switching off the furnace to achieve the maximal cooling rate. The samples were washed with water and characterized by XRD using an in-house Panalytical X'pert X-ray diffractometer with Cu Kα radiation (λ = 1.5406 Å).

**In Situ Synchrotron Characterization.** An in situ synchrotron radiation X-ray diffraction study was performed using MAX phase and chloride salt powders premixed in agate mortar in different molar proportions inside a nitrogen-filled glovebox (to avoid salt hydration by air moisture) and loaded into Hilgenberg special glass capillaries. Capillaries were loaded with the powder mixture only for about 1/3 of the length to keep some space for evolving gases and for the condensation of reaction products in the cold zone. Material-filled capillaries were taken out of the glovebox and briefly exposed to air for torch sealing.

The samples were heated inside sealed capillaries typically up to 853–873 K, kept at this temperature for several hours, and cooled down. 2D XRD images were continuously recorded during the whole heating/cooling cycle during several hours lasting experiments. Heating guns with somewhat different hotspot sizes were used in experiments performed at ESRF and at MAX IV synchrotron facilities; in both cases, the size of the cold zone was larger than the size of the hot zone. In this case, gaseous reaction products are mostly removed into an empty cold zone due to the process of reaction condensing on the walls of the capillary.

In situ synchrotron characterization of MXene synthesis was conducted at the Rossendorf Beamline (BM20) of the European Synchrotron Radiation Facility (ESRF)<sup>9</sup> using a radiation wavelength (λ) of 0.7381 Å. A hot air gas blower (FMB Oxford DGB-1) was used for the annealing experiments. Experimental data were collected in transmission geometry using a DECTRIS PILATUS3 X 2M detector and analyzed with Bubble software.<sup>32</sup> The detector geometry parameters were calibrated using the NIST (SRM660x) LaB<sub>6</sub> standard with PyFAI software. The samples were heated typically up to 823–873 K with a heating rate of 2–5 K/min, and 60 s scans were recorded continuously.

The second part of the experiments was conducted at the DanMAX beamline at the MAX IV Laboratory. The temperature was controlled by using a HyBec Hi-Heater controlled by a Lakeshore 336 temperature controller. The diffraction data were collected up to a maximum 2θ of 28° (Q<sub>max</sub> ~ 3.75 Å<sup>-1</sup>) using a wavelength of 0.8266 Å (15.00 keV) on a DECTRIS PILATUS3 X 2M CdTe area detector. The wavelength and geometry of the instrument were refined using a LaB<sub>6</sub> standard (NIST SRM660c) and the program PyFAI.<sup>33</sup> The raw area detector data were azimuthally integrated to intensity versus 2θ
using the MatFRAIA algorithm.³⁴ The maximum temperature studied in these experiments was 823 K, with a heating rate of 5 K/min, and each scan lasted for 1 s.

The Ti₃C₂Cl₂ structure model was built by rescaling the known Ti₃C₂ structure to the experimentally determined unit cell and terminating the coordination spheres of Ti atoms with Cl. Atomic coordinates were not refined against XRD data, since a meaningful Rietveld refinement is not possible for given data quality (too few reflections and multiphase sample).

## ■ RESULTS

The main objective of this work was to study in situ the reaction of MAX phase etching in molten salts, leading to formation of MXenes. These experiments aimed to reveal the structure of pristine MXene still immersed in molten salts. The structure of Ti-MXene prepared by the LAMS method was so far reported only after cooling and water washing.²⁴,²⁷,³⁵ Structural analysis of phases evolving in the process of MAX phase etching and MXene formation was performed by using synchrotron radiation XRD data for the two systems. Etching of the MAX phase in molten ZnCl₂ is presented first, followed by results obtained for SnCl₂ etching.

### Reaction of the MAX Phase with Molten ZnCl₂.
An in situ synchrotron radiation X-ray diffraction study was performed using Ti₃AlC₂ MAX phase and ZnCl₂ powders premixed in different molar ratios, loaded into sealed glass capillaries, and heated above the melting point of the salt for periods up to 6 h. The best results were obtained for the sample premixed in a 1:6 molar ratio of Ti₃AlC₂/ZnCl₂ in agreement with earlier ex situ studies.¹⁸,²⁰

The reference XRD pattern recorded from Ti₃AlC₂ powder showed the high purity of the precursor material with unit cell parameters refined using 21 reflections in the P6₃/mmc space group (a = 3.071 Å and c = 18.577 Å) in good agreement with literature data.¹⁰,²⁰ Experimental and theoretically simulated patterns of Ti₃AlC₂ are compared in Figure S2 in the SI file.

XRD patterns recorded from a mixture of Ti₃AlC₂ and ZnCl₂ powders (mixed in a 1:6 molar proportion) are dominated by a large number of very strong reflections from ZnCl₂ (Figures 1 and S2 in the SI file). The mixed Ti₃AlC₂/ZnCl₂ sample was rapidly heated to 853 K and kept at this temperature for ~280 min, while 2D XRD images were continuously recorded every 60 s, thus providing detailed time-resolved characterization of phases formed in the process of the Ti₃AlC₂ reaction with molten ZnCl₂.

Above the melting point, all reflections from crystalline ZnCl₂ disappeared (Figures S2 and S3), and a new broad feature from the liquid salt appeared (Figure S2 in the SI file). The XRD pattern recorded just above the melting point of salt at 590 K (Figure S3) still shows only reflections of the precursor Ti₃AlC₂ with unit cell parameters a = 3.083 Å and c = 18.635 Å, which are only slightly higher than those at ambient temperature due to thermal expansion.

Further heating of the MAX phase in molten ZnCl₂ resulted in dramatic changes in the sample appearance and structure. Intensive bubbling was observed in the heating zone of the capillary above the melting point of ZnCl₂ as the temperature approached 853 K, correlating with a significant drop in the intensity of all XRD reflections (Figure 1).

The strongest reflections of the MAX phase were still observed at the moment of intense bubbling but dropped in intensity rather significantly. This bubbling obviously disturbs data recording due to the movement of the powder and liquid enclosed in the capillary. However, as will be discussed below,

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<figure><img src="image_13.png" /><figcaption>**Figure 1.** (a) Selected XRD patterns recorded from the sample of ZnCl₂ and Ti₃AlC₂ premixed in a 6:1 mole ratio and annealed at 853 K (λ = 0.73804 Å). Every 10th pattern is shown focusing on the temperature region of liquid ZnCl₂ (see Figures S2 and S3 in the SI file for the data recorded just below and above the temperature point of ZnCl₂ melting during heating). (b) Low-angle part of XRD patterns showing (002) reflections of precursor Ti₃AlC₂ and MXene formed by the reaction with ZnCl₂. Some background was subtracted from all patterns.</figcaption></figure>

this initial stage of the reaction also results in a true loss of MAX phase structure ordering. The bubble formation slowed down after about 20 min of annealing at 853 K, and a new set of reflections emerged (Figure 1a).

The formation of MXene at this moment is clearly evidenced by the appearance of the (002) reflection with a d-spacing of 11.1 Å (Figure 1b). The etching reaction can be easiest followed by observing the increase in the intensity of the broader (002) reflection from the MXene (d = 11.1 Å) over the time of the reaction, while the intensity of the relatively sharp (002) reflection from the precursor MAX phase (~9.4 Å) fades (Figures 1b and 2c).

The intensity of the (002) reflection of the MAX phase faded sharply at the initial stage of the reaction and disappeared nearly completely after ~3 h of annealing in ZnCl₂ at 853 K (Figures 1 and 2a). The d(002) value of the MAX phase increased linearly due to the thermal expansion at
<figure><img src="image_14.png" /><figcaption>**Figure 2.** Time evolution of *d*(002) for the precursor MAX phase (a) and *d*(002) for MXene (b) recorded during the annealing experiment performed for sample premixed in a 1:6 proportion of Ti₃AlC₂/ZnCl₂. The area ratio for (002) reflections of the MXene/MAX phase (c).</figcaption></figure>

temperatures below the melting point of ZnCl₂, but it also continued to increase slightly (by ~0.04 Å) for about 1 h at 853 K (Figure 2a). The slight expansion of the MAX phase structure correlates with a very strong drop in the intensity of

the (002) reflection and intense bubble formation inside the capillary observed visually. It is important to note that the remaining traces of the crystalline MAX phase still correspond to Ti₃AlC₂ rather than Zn-substituted Ti₃ZnC₂ (Figure S3 in the SI file).

The MXene (002) reflection first appears after the end of the intense bubbling period (time region from 100 to ~225 min in Figure 2c). The *d*(002) value of MXene slightly increases in the initial stage of annealing but remains almost unchanged at ~11.15 Å over the next 3 h. The intensity of the (002) reflection increases during the first 3 h of annealing and remains nearly the same after longer annealing, indicating complete MAX phase conversion into MXene (Figure 2b). The integral peak intensity ratio of MXene(002)/MAX(002) reflections showed a linear increase during annealing at 853 K (Figure 2c).

A detailed analysis of the XRD pattern recorded at the end of the heating is shown in Figure 3. The analysis of this pattern shows that all major reflections are not from the precursor MAX phase and can be assigned to the newly formed MXene

<figure><img src="image_15.png" /><figcaption>**Figure 3.** (a) XRD pattern recorded directly in molten salt after ~215 min of annealing at 853 K. For this figure, background subtraction also included the feature originating from molten ZnCl₂. The samples of the MAX phase and ZnCl₂ were premixed in a 1:6 molar ratio in this experiment (see also Figure 1). Radiation wavelength λ = 0.73804 Å. (b) Structure model of Ti₂C₂Cl₂ simulated using experimental unit cell parameters.</figcaption></figure>
structure. See Figure S3 in the SI file, where the XRD pattern of the MAX phase recorded directly after the melting of the ZnCl₂ is plotted together with the XRD pattern recorded at the end of annealing.

Notably, the structure of MXene found in our experiments shows relatively good ordering of 2D titanium carbide layers. This is in strong contrast to the structure of Ti-MXene synthesized using HF-related solution routines, which typically demonstrates the absence or very poor ordering of 2D layers. Typically, XRD patterns of Ti₃C₂Tₓ prepared using HF etching show a strong 00l- set of reflections and rather broad features due to other lattices, most likely due to the presence of a variety of terminating functional groups (Tₓ) with x corresponding to -F, -OH, and possibly some other groups.¹⁰,¹¹,¹⁷,³⁶ That is not the case for Ti₃C₂Cl₂, which exhibits about 15 relatively sharp XRD reflections besides the 00l- set. The most likely reason for better ordering is the absence of terminating groups other than chlorine. Considering reports of pure Cl termination of LAMS MXene, the ideal composition Ti₃C₂Cl₂ of this MXene has been considered in earlier studies.²⁵ However, experimental materials are likely to deviate from the perfect stoichiometry; thus, it is more relevant to assume Ti₃C₂Clₓ composition.

The set of new strong reflections that appear after the reaction of the MAX phase with molten ZnCl₂ is interpreted as MXene Ti₃C₂Clₓ and indexed using unit cell parameters *a* = 3.186 Å and *c* = 22.312 Å (*P*6₃/*mmc*), Figure 3a. Note that the *a*-parameter of the MXene structure is about 0.1 Å larger compared to the *a*-parameter of the precursor Ti₃AlC₂. The unit cell parameters calculated for MXene formed in our experiments agree with a structural model of Ti₃C₂Cl₂ constructed in our study, assuming complete termination of the 2D layers with chlorine (Figure 3b). A simulated XRD pattern for the Ti₃C₂Cl₂ structure shown in Figure 3b shows reasonable agreement with the experimental pattern (Figure S6) considering the multiphase nature of the sample, which also includes TiC and some peaks likely originating from the remaining disordered MAX phase.

The pattern shown in Figure 3 shows only a rather weak (002) reflection from the MAX phase and a relatively strong set of reflections from TiC. The formation of TiC can be explained if some Ti₃C₂ layers formed by etching away Al are not immediately terminated by chlorine instead of forming a Ti₃C₂Clₓ restack into a titanium carbide lattice.

Possibly the most surprising finding of our study is the absence of MXene swelling in molten ZnCl₂. Prior to experiments, we anticipated that the structure of Ti₃C₂Clₓ would be found in our experiments in a ZnCl₂ melt-swollen state, expanded in the *c*-direction compared to ambient water-washed and solvent-free MXene. A similar HF-swollen structure of pristine MXene was found in our recent experiments with HCl + LiF synthesis.¹⁷ The interlayer distance of pristine MXene recorded directly in HF solution was found to be expanded by ~3 Å compared to the standard MXene studied ex situ in a solvent-free state. However, the *c*-unit cell parameter of Ti₃C₂Clₓ recorded in situ (22.33 Å) at 853 K in a melt-immersed state appeared to be nearly the same as in earlier ex situ (22.24 Å) studies performed with water-washed and solvent-free materials.²⁰ Essentially the same *c*-unit cell parameter was found also in our experiments after cooling the sample below the melting point of ZnCl₂ (Figure S5 in the SI file) and further cooling down to ambient temperature. Therefore, no pristine MXene is found in molten salt synthesis.

It is also interesting to note the absence of XRD reflections from metallic Zn in all XRD patterns recorded in our in situ experiments (Figure 1). The temperature of annealing is above the melting point of Zn (695.5 K), but no reflections from metallic zinc were observed even after cooling below the melting point and even after ZnCl₂ solidification.

The analysis of the data recorded during the transformation of the MAX phase into MXene during annealing in molten ZnCl₂ indicates the absence of other crystalline intermediate phases. For example, we do not find clear evidence for the formation of Ti₃ZnC₂, which was earlier found in experiments with small loading of ZnCl₂.²⁰ Figure S3 shows the comparison of theoretical XRD patterns of Ti₃ZnC₂, Ti₃AlC₂, and experimental patterns of the MAX phase recorded during initial stages of etching prior to the appearance of MXene reflections.

However, the strong drop in the intensity of the MAX phase reflections at the initial stages of annealing (~20 min, Figure 1) can be considered an indication of the appearance of an intermediate phase with a strongly disordered structure.

The very strong drop in the intensity of MAX phase reflections prior to the emergence of MXene could be interpreted in two ways. The most trivial explanation suggests that the changes in intensity observed during the first 10–20 min of annealing are simply a result of bubbling and a related disturbance of data recording. The second explanation suggests a loss of long-range order or nearly complete delamination of Ti₃C₂ layers due to the etching of Al and formation of gaseous AlCl₃. In the latter case, the drop in the intensity of MAX phase reflections would also still be observable in ex situ experiments for the sample rapidly cooled after short annealing at 853 K.

Therefore, additional synthesis experiments were performed using bulk powder samples and ex situ XRD characterization to verify the structure of materials formed at the initial stage of the etching reaction. Two samples of a Ti₃AlC₂/ZnCl₂ mixture (1:6) were annealed at 853 K for short and long periods of time in sealed tubes to make the conditions of synthesis as close as possible to in situ experiments. The first sample was kept at the maximal temperature of 853 K for only 1 min, thus minimizing annealing time, which also includes heating (8°/K) and rapid cooling by switching off the heating with ZnCl₂ remaining in a molten state for about 15 min. The second sample was annealed at 853 K for 3 h, which is sufficient for a nearly complete transformation of the MAX phase sample into MXene, according to the in situ data shown in Figure 1. Both samples were washed with water to remove excess ZnCl₂, and XRD patterns were recorded at ambient temperature on air (Figure 4).

The sample prepared by annealing Ti₃AlC₂ in a molten ZnCl₂ for a longer time (3 h) shows strong reflections from MXene Ti₃C₂Clₓ and reflections from metallic zinc, both in good agreement with literature data (Figure 4).²⁰ The sample annealed in molten ZnCl₂ for a short time (1 min at a maximal temperature of 853 K) showed only traces of (002) reflections from both the MAX phase and MXene. However, the sample shows several broader reflections, suggestively belonging to Ti₃C₂ layers, and relatively strong reflections identified as metallic Zn.

The data shown in Figure 4 suggest that the drop in the intensity of major XRD reflections of the MAX phase observed in situ at the initial stage of annealing (Figure 1) is due to a true disruption of the material's structure and nearly complete
<figure><img src="image_16.png" /><figcaption>**Figure 4.** XRD patterns recorded using a conventional diffractometer (Cu Kα radiation) from a bulk sample of Ti₃AlC₂ and two samples of Ti₃AlC₂/ZnCl₂ mixture (1:6 molar) heated rapidly up to 853 K and annealed for 1 min and 3 h. The heating time with a rate of 8 °C/min adds about 30 min of exposure of the MAX phase to ZnCl₂ melt, and as fast as possible cooling of the furnace adds about 15 min required for melt to solidify. The samples were rapidly cooled back to ambient temperature to quench the high-temperature phases. Reflections due to metallic Zn are marked by (♦) symbols.</figcaption></figure>

disorder of the 2D layers packing rather than the effect of bubble formation and turbulence inside the capillary.

Moreover, ex situ data suggest that metallic zinc is formed in these samples only after dissolving the excess of ZnCl₂ by washing with water. Metallic Zn was not found in our in situ experiments during both annealing and crystallization of the salt due to cooling. Therefore, most of the Zn formed as a reaction product between Al and ZnCl₂ should be dissolved in the molten salt. The relatively high intensity of Zn reflections indicates that the etching reactions proceeded rather rapidly. About 10–20 min of annealing with only 1 min at the maximum temperature of 853 K was sufficient for etching away most of the Al (removed as AlCl₃ gas) while Zn dissolved in the ZnCl₂ melt.

Analyses of XRD data recorded in situ (Figure 3) and ex situ (Figure 4) indicate that delaminated 2D sheets can be considered as intermediate states between the MAX phase and MXene. Most of the Al is likely to be already removed from the MAX phase and most of the Zn is dissolved in the molten salt in the delaminated state. XRD data also indicate that the “delaminated phase” is only partly converted into MXene after 3 h of annealing. The XRD pattern shown in Figure 3 is likely to include some reflections from the delaminated phase not restacked into the ordered MAX phase, as indicated, e.g., by a broader and stronger (105) reflection than expected (see Figure S6, where the experimental XRD pattern and simulated pattern of Ti₃C₂Cl₂ are compared).

In situ experiments presented here provide insights into the mechanism of MXene synthesis by the LAMS method (see the Discussion section). It should be mentioned with caution for future experiments that some additional reaction products were found in two less successful experiments due to the partial reaction of materials with water released from easily hydrated ZnCl₂ inside the capillary. Water is very easily

absorbed by ZnCl₂ even after a short exposure to air. This water is released after heating due to the thermal decomposition of the hydrates, and then it moves and condenses in a cold zone of capillary prior to the melting of salt. However, when a larger diameter heating gun (and a smaller cold zone) was used, for the samples premixed in proportions of 1:6 and 1:9, experiments showed incomplete transformation of the MAX phase into MXene and the appearance of additional undesirable reaction products. The new “parasitic” phase appeared in these experiments shortly (15 min) after the melting of ZnCl₂ with the strongest characteristic reflections at 8.372, 3.377, and 2.765 Å (Figure S8). These reflections were indexed as (001), (101), and (011) of TiOCl. Using five strong reflections, the unit cell parameters of this phase were calculated as *a* = 3.688 Å, *b* = 3.381 Å, and *c* = 8.368 Å (Pmmm), which are slightly higher than ambient temperature values of TiOCl³⁷ due to thermal expansion of the material. It is known that TiOCl can be formed by the reaction of titanium oxide with titanium chloride at temperatures close to our experiments.³⁸ Formation of TiOCl was not observed in experiments with a larger heating gun only for the sample with a smaller ZnCl₂ loading (1:3) (Figure S9), confirming that the formation of TiOCl is related to water included in partly hydrated ZnCl₂. However, only a relatively small part of the MAX phase was converted to MXene in this experiment due to an insufficient amount of molten ZnCl₂. Notably, the same structure of MXene was found even in the experiments where the additional TiOCl phase was formed.

It can be concluded that using water-free salts and a sufficiently large “cold zone” for the condensation of reaction products is an important condition for the complete conversion of the MAX phase into MXene in sealed capillaries. Incomplete conversion of the MAX phase into MXene and the appearance of undesirable side products can be avoided.

### In Situ Study of the MAX Phase Reaction with SnCl₂.

The reaction of the MAX phase with molten SnCl₂ was studied in situ by XRD using the same procedure as in experiments with molten ZnCl₂ described above. Samples were mixed in molar ratios 1:3, 1:6, and 1:9 (MAX phase/SnCl₂) and annealed at 853–873 K for 6 h.

Similar to experiments with ZnCl₂, vigorous evolution of gas bubbles (AlCl₃) was observed in the initial stages of the reaction, and at the same time, the color of the sample changed from black to brown (see Figure S11 and Video in the SI). However, complete transformation was not achieved within 6 h in experiments with etching the MAX phase with molten SnO₂, even in the experiment with the highest salt loading molar ratio (1:9) and a temperature of 873 K (Figure 5).

Experiments with a smaller amount of loaded SnCl₂ and a lower temperature of 853 K showed also slower kinetics of the reaction and less complete conversion of Ti₃AlC₂ into MXene. This result is in agreement with an earlier ex situ study of MAX phase etching with SnCl₂ where incomplete conversion was observed after annealing for 8 h at 873 K.²¹ However, unlike ex situ results,²¹ no metallic Sn was detected in our XRD experiments (similar to the absence of Zn in experiments with ZnCl₂) even after cooling and solidification of the melt, suggesting that Sn dissolves in molten SnCl₂. The detailed analysis of XRD data is complicated by significant overlap of reflections from MXene, the remaining MAX phase, and the TiC phase. Nevertheless, the unit cell parameters of MXene Ti₃C₂Clₓ produced by the etching of the MAX phase with SnCl₂ for the pattern recorded at the end of annealing at 873 K
<figure><img src="image_17.png" /><figcaption>**Figure 5.** Selected XRD patterns (λ = 0.73804 Å) recorded during the in situ experiment with MAX phase etching by molten SnCl₂. The MAX phase and SnCl₂ were mixed in a 1:9 proportion (molar ratio) and annealed at 873 K.</figcaption></figure>

(a = 3.18 Å and c = 22.287 Å, P6₃/mmc) appeared to be in remarkably good agreement with the experiments involving ZnCl₂ etching.

Similar to the experiments with ZnCl₂, annealing of the MAX phase in molten SnCl₂ goes through the stage of intense bubbling correlated with loss of ordering of Ti₃C₂ layers (Figure 5), suggesting the formation of a delaminated phase intermediate between the MAX phase and MXene. The MXene phase starts to emerge after about 1 h of annealing at 873 K (Figure 6b). Formation of other intermediate phases was not observed, similar to the experiment with ZnCl₂.

The reaction can be followed by the change in the relative intensity of (002) reflections of the MAX phase and MXene over the whole period of annealing (Figure 6c). The *d*(002) values of the MAX phase and MXene remain almost unchanged during annealing. Small shifts observed during heating are due to thermal expansion, as evidenced by the reverse change recorded during cooling (Figure 6a,b). The relative integral intensity of MXene(002)/MAX(002) reflections showed an approximately linear increase during annealing at 873 K (Figure 6c). It is obvious that the conversion of the MAX phase into MXene still continues after 6 h of annealing at 873 K (Figure 6c).

Extrapolating the time dependence shown in Figure 6c, it can be suggested that complete conversion of the MAX phase into MXene could possibly be achieved after prolonged (12–18 h) annealing. The slow kinetics and limitations of synchrotron time available for long experiments prevented us from obtaining pure MXene in this experiment.

Summarizing this part, experiments with the MAX phase in molten SnCl₂ showed very similar results to the ones in ZnCl₂. The same MXene phase was formed by etching the MAX phase with ZnCl₂ and SnCl₂ but the kinetics of the reaction was slower. Experiments with Ti₃AlC₂ etched with SnCl₂ reveal the same mechanism of the MXene formation from the MAX phase, which includes loss of ordering of Ti₃C₂ layers and formation of the “delaminated” phase as an intermediate product. Restacking of the chlorine-terminated 2D layers of the delaminated phase into Ti₃C₂Clₓ MXene occurs during longer

<figure><img src="image_18.png" /><figcaption>**Figure 6.** Evolution of (002) reflections from the MAX phase and MXene over the heating–cooling experiment in molten SnCl₂ (selected XRD patterns shown in Figure 5). The experiment includes heating up to 873 K, annealing at this temperature, and cooling back to ambient temperature.</figcaption></figure>

annealing. The Ti₃C₂Clₓ phase shows no swelling in molten SnCl₂ and remains unchanged after cooling and water washing.

## DISCUSSION

The experiments presented above provide a detailed time-resolved characterization of structural transformations between the precursor MAX phase and MXene evaluated in situ directly
<figure><img src="image_19.png" /><figcaption>Figure 7. Schematic diagram illustrating the mechanism of the Ti₃AlC₂ reaction with molten ZnC₂ and formation of chlorine-terminated Ti₃C₂Clₓ MXene.</figcaption></figure>

in molten ZnCl₂ and SnCl₂. The data allow us to confirm the main points previously reported in ex situ studies²⁰,²²,²⁵,³⁵ and provide insights into the mechanism of MAX phase etching in molten salts.

The main experimental observations reported above can be summarized as follows.

* The disordered delaminated phase is found during the first stage of Ti₃AlC₂ annealing (~10–20 min) in both ZnCl₂ and SnCl₂. This phase can be considered an intermediate between Ti₃AlC₂ and MXene with 2D layers in a disordered state. This observation is in agreement with the only previously published in situ study of Nb₂GaC etching in molten salts, which reported complete amorphization of their materials at the initial stages of etching with the MXene structure emerging after prolonged annealing.³⁰

* Rapid formation of gaseous AlCl₃ visually observed inside the capillary at the initial stage of the reaction is accompanied by the dissolution of Zn in molten ZnCl₂. Crystallization of metallic Zn occurs only when the salt is dissolved by water after cooling and melt solidification.

* The delaminated Ti₃C₂ layers are terminated by chlorine and restacked into the MXene structure. The first reflections of MXene appeared after ~15–20 min of annealing at 853 K (for ZnCl₂), and the reaction is mostly completed after ~3 h.

* No swelling of Ti₃C₂Clₓ was found in both molten ZnCl₂ and SnCl₂. Molten salts cannot penetrate between Cl-terminated Ti₃C₂ layers. The MXene structure found directly in molten salts is the same as the one observed after cooling and water washing at ambient temperature (except for the effects of thermal expansion).

* Crystallization (restacking) of Ti₃C₂Clₓ in the molten ZnCl₂ explains the better ordering of the MXene structure as compared to MXenes produced using solution-based methods.

* No other crystalline intermediate phases were observed in our in situ experiments. Experimental data revealed only reflections of Ti₃AlC₂, TiC, and MXene.

* Annealing in both ZnCl₂ and SnCl₂ showed qualitatively very similar reaction pathways providing the same Cl-terminated MXene phase in the end, in agreement with ex situ studies performed using a variety of etching salts.¹⁸,²⁰,²⁴,²⁵,²⁷,³⁹

These results provide insights into the mechanism of MAX phase-to-MXene conversion by annealing in molten salt.

According to the mechanism of MXene synthesis proposed in earlier studies, Al is etched away from Ti₃AlC₂ by the reaction with molten salt and evaporates as AlCl₃ gas with simultaneous termination of titanium carbide layers by chlorine and formation of elemental zinc.²⁰,²⁶ The synthesis of MXene is described by the following reactions.²⁶

$$2\text{Ti}_3\text{AlC}_2\text{(s)} + 3\text{ZnCl}_2\text{(liq)} \rightarrow 2\text{Ti}_3\text{C}_2\text{(s)} + 2\text{AlCl}_3\text{(g)} + 3\text{Zn(s)} \quad (1)$$

$$2\text{Ti}_3\text{C}_2\text{(s)} + 2\text{ZnCl}_2\text{(liq)} \rightarrow 2\text{Ti}_3\text{C}_2\text{Cl}_2\text{(s)} + 2\text{Zn(s)} \quad (2)$$

In situ experiments presented in our study confirm these reactions but add significant insights to the mechanism of etching illustrated in Figure 7. The removal of Al from the MAX phase structure is found to be assisted by delamination of the Ti₃AlC₂ structure at step (1) of the reaction, evidenced by almost complete amorphization of the material. Delamination suggestively helps to expose all surfaces of 2D sheets to the ZnCl₂ melt, which makes reaction step (2) possible.

The delamination is likely to occur due to formation and rapid escape of gaseous AlCl₃, “exploding” the MAX phase structure. Therefore, the high vapor pressure of AlCl₃ formed much above its boiling point (453 K) at 850 K is likely to assist in the delamination of the MAX phase structure. In principle, the delamination can also be considered as extreme swelling of the material in the molten salt.

Analysis of our data suggests that once delaminated, the Ti₃C₂ sheets get rapidly terminated by chlorine, while Al escapes in the form of gaseous AlCl₃, and elemental Zn (or Sn) is dissolved in the molten salt.⁴⁰ Note that metallic zinc was not found in our experiments, even after cooling and salt solidification. Most likely, the absence of metallic Zn prior to water washing is explained by the previously reported formation of a Zn-enriched glass phase after rapid cooling of the ZnCl₂ melt with dissolved Zn.²⁰,⁴⁰ Removing Al from the MAX phase interlayers with simultaneous termination of Ti₃C₂ layers with chlorine seems to be in agreement with in situ data collected in our study.

The MXene structure is found to appear after ~20 min of annealing of the disordered delaminated phase and is suggestively formed by the restacking of Cl-terminated Ti₃C₂ flakes. The molten salt is expelled from the structure of MXene during the restacking, as evidenced by the absence of swelling.

The absence of the melt-swollen MXene phase is a nontrivial result considering that the XRD was recorded in our experiments from the MXene immersed in liquid molten ZnCl₂ (or SnCl₂). That is in strong contrast to our recent in situ studies of solution synthesis, which demonstrated the
existence of the MXene structure with the c-lattice expanded by ~3 Å due to swelling in LiCl + HF solution (essentially in HF acid).¹⁷ Thanks to swelling, HF solutions are capable of penetrating into interlayers of MXene. The etching reaction then proceeds by slow conversion of the MAX phase with MXene propagating along the interlayers with both phases coexisting for prolonged periods of time.¹⁷

The swelling of MXene in water is related to -OH termination of Ti₃C₂ layers.¹³,³¹,⁴¹ Therefore, swelling of Cl-terminated MXene in molten chloride salts could also be expected following the general “like dissolves like” principle.

The absence of MXene swelling in molten ZnCl₂ and SnCl₂ indicates a different mechanism of the MAX phase etching reaction rather than slow etching along narrow interlayers. The MXene phase appears in our experiments only after nearly complete amorphization of the MAX phase, which is suggestively related to delamination of Ti₃C₂ layers.

The molten salt is not capable of penetrating between Cl-terminated layers of MXene, thus excluding the possibility of slow etching starting from the entrance points of interlayers and propagating slowly toward the inner parts. In the absence of MXene swelling in molten salts, formation of the delaminated phase is the only way to provide salt access to the whole surface of Ti₃C₂.

The reaction diagram shown in Figure 7 does not include the TiC side product, which is very commonly found in LAMS synthesis.²⁰ This phase is also evident in the in situ experiments presented here (Figure 3a). No other intermediate phases were found in our experiments.

Some earlier studies assumed the Zn-substituted phase as the “intermediate” phase in the reaction, leading to MXene formation.¹⁹ However, formation of Zn-substituted MAX phase Ti₃ZnC₂ was reported only for experiments with a much smaller loading of ZnCl₂ (1:1.5 molar ratio).²⁵ Our experiments show no evidence for the presence of crystalline Zn- and Sn-substituted MAX phases in the reaction of Ti₃AlC₂ with molten salts loaded in larger amounts (1:6 and 1:9 molar ratios). A large excess of molten salt is expected to assist the reaction by more complete delamination of the MAX phase and by dissolution of Zn.²⁰ However, we cannot completely rule out the possible presence of the Zn-substituted phase if it is present in an amorphous state.

The Al substitution by Zn with the formation of crystalline Ti₃ZnC₂ seems to be unlikely due to Zn solubility in molten ZnCl₂. Dissolving Zn from the interlayers of the crystalline Ti₃ZnC₂ phase would require expansion of the c-lattice (swelling) to allow penetration of liquid salt inside the lattice interlayers. However, experimental data demonstrate that the absence of swelling and Ti₃ZnC₂ would be stable in a molten salt. Therefore, Zn- or Sn-substituted MAX phases can be considered as a side product (similar to TiC) rather than an intermediate phase.¹⁹ The absence of intermediate Al-substituted MAX phases is in agreement with earlier ex situ studies, which reported that formation of Zn-substituted MXene is unfavorable in experiments with a larger excess of molten ZnCl₂.²⁰

## ■ CONCLUSIONS

A time-resolved in situ study of MXene formation by MAX phase etching in molten salts was performed here using synchrotron radiation XRD. The main conclusions of the study are the following.

* The structure of MXenes formed by etching of the MAX phase with ZnCl₂ and SnCl₂ shows no swelling in molten salts. The MXene Ti₃C₂Clₓ structure found in situ in the molten salt (a = 3.186 Å and c = 22.312 Å, P6₃/mmc) and after cooling and salt solidification is the same except for the effects of thermal expansion.

* The absence of Ti₃C₂Clₓ swelling in molten salts is a nontrivial result. It shows that molten salts are expelled from the structure at the moment of MXene formation and are not capable of penetrating between chlorine-terminated Ti₃C₂ layers.

* That is in strong contrast to synthesis in aqueous solutions, where pristine MXene with a structure swollen in HF solution is formed during Al etching.¹⁷ In the absence of swelling, the formation of MXene is not possible by gradual removal of Al from MAX phase interlayers starting at the edges of flakes and propagating into the deeper parts of interlayers.

* MXene formation occurs through an intermediate disordered delaminated phase formed at the initial stage of annealing in the molten salt.

* The termination of Ti₃C₂ layers by chlorine suggestively occurs in the disordered delaminated phase simultaneously with AlCl₃ formation and Zn dissolution in the molten salt. No other intermediate phases were found.

* The crystalline ordered MXene structure suggestively forms in the molten salt by restacking of 2D Ti₃C₂Clₓ layers present in the delaminated disordered intermediate state.

* The in situ data suggest that Zn is dissolved in molten ZnCl₂. Metallic Zn (or other metals) typically found in ex situ studies²⁰,²⁴ is not found prior to water washing.

In the future, it will be interesting to expand the in situ XRD experiments conducted in this study for molten salt etching of Ti₃AlC₂ to include other MAX phases and MXenes. Moreover, in situ studies of several other MXene synthesis procedures could contribute to the understanding of similarities and differences in the mechanism of MAX phase etching.

## ■ ASSOCIATED CONTENT

### ■ Supporting Information

The Supporting Information is available free of charge at https://pubs.acs.org/doi/10.1021/acs.chemmater.4c02989.

Additional data and figures including indexing of the experimental Ti₃AlC₂ precursor; comparing the structure of the MAX phase prior to and after melting of ZnCl₂ at temperatures up to 853 K; experimental XRD patterns of the MAX phase and MXene compared to theoretically simulated patterns for Ti₃AlC₂, Ti₃ZnC₂, and Ti₃C₂Cl₂; additional data recorded in experiments with SnCl₂ etching; scanning electron microscopy (SEM) images of MXene samples; and optical images (PDF)

Video showing bubbles evolving inside of the sealed capillary at the initial stages of the etching reaction (MP4)

## ■ AUTHOR INFORMATION

### Corresponding Author

Alexandr V. Talyzin – Department of Physics, Umeå University, Umeå SE-901 87, Sweden; ● orcid.org/0000-0002-3320-8487; Email: alexandr.talyzin@umu.se
## Authors

**Artem Iakunkov** – School of Engineering Sciences in Chemistry, Biotechnology and Health, KTH Royal Institute of Technology, Stockholm SE-100 44, Sweden

**Nicolas Boulanger** – Department of Physics, Umeå University, Umeå SE-901 87, Sweden

**Bartosz Gurzeda** – Department of Physics, Umeå University, Umeå SE-901 87, Sweden

**Gui Li** – Department of Physics, Umeå University, Umeå SE-901 87, Sweden

**Christoph Hennig** – Institute of Resource Ecology, Helmholtz Zentrum Dresden Rossendorf, 01328 Dresden, Germany; The Rossendorf Beamline, European Synchrotron Radiation Facility, Grenoble 38043, France; ● orcid.org/0000-0001-6393-2778

**Volodymyr Svitlyk** – Institute of Resource Ecology, Helmholtz Zentrum Dresden Rossendorf, 01328 Dresden, Germany; The Rossendorf Beamline, European Synchrotron Radiation Facility, Grenoble 38043, France; ● orcid.org/0000-0001-5449-8009

**Mads Ry Vogel Jørgensen** – Department of Chemistry and INANO, Aarhus University, DK-8000 Aarhus C, Denmark; MAX IV Laboratory, Lund University, Lund 22594, Sweden; ● orcid.org/0000-0001-5507-9615

**Innokenty Kantor** – Department of Chemistry and INANO, Aarhus University, DK-8000 Aarhus C, Denmark

**Igor A. Baburin** – Earth and Environmental Sciences, Ludwig-Maximilians-Universität München, 80333 München, Germany

**Mahiar Max Hamedi** – School of Engineering Sciences in Chemistry, Biotechnology and Health and Wallenberg Initiative Materials Science for Sustainability, KTH Royal Institute of Technology, Stockholm SE-100 44, Sweden; ● orcid.org/0000-0001-9088-1064

Complete contact information is available at: https://pubs.acs.org/10.1021/acs.chemmater.4c02989

## Notes

The authors declare no competing financial interest.

## ACKNOWLEDGMENTS

Energimyndigheten (grant number No. 50620-1) is acknowledged for financial support Kempestiftelserna are acknowledged for stipend funding to employ B.G. The authors acknowledge the ESRF and CRG support for providing synchrotron radiation beam time (experiment number: A20-1-860) on the BM20 (ROBL) beamline. The authors acknowledge MAX IV Laboratory for time on Beamline DanMAX under Proposal 20221405. Research conducted at MAX IV is supported by the Swedish Research Council under contract 2018-07152, the Swedish Governmental Agency for Innovation Systems under contract 2018-04969, and Formas under contract 2019-02496. DanMAX is funded by the NUFI grant no. 4059-00009B. A.I. and M.M.H. acknowledge the Swedish Energy Agency (Energimyndigheten) for funding this project (grant number 48489-1).

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