Solubility of MoS2 and Graphite in Molten Salt: Flowers, Faceted Crystals, or Exfoliation?

2D materials are of interest in various applications such as energy conversion, storage, and sensing. These materials are prepared by bottom‐up or top‐down methods that are difficult to control and suffer from low yield. Synthesis in molten salt is suggested as an alternative in which the balance between exfoliation and solubility is explored. It is demonstrated that when a pellet of 2D material is insoluble in molten salt (graphite in any salt), it is exfoliated. For low solubility (MoS2 in NaCl/KCl), the 2D material nucleates and grows into a small flowerlike structure composed of thin MoS2 sheets through Ostwald ripening. For high solubility in the molten salt (MoS2 in CsCl), it forms larger flowers. Herein, the molten salt treatment of high and low surface area MoS2 (micron‐size particles and a single large pellet, respectively) is compared. The particles yielded facet MoS2 crystals through dissolution–nucleation–recrystallization process and the pellet yielded flowers. Herein, methods for synthesizing 2D materials with controllable size and shape are promoted by simple molten salt treatment that opens an avenue to the development of soluble (MoS2) and non‐soluble (graphite) materials with different morphologies in a simple and affordable way.


Introduction
The 2D materials have recently received attention owing to their unusual electronic structures and exceptional physical properties. [1,2]5] (%1000 °C) and long treatment time (10 days), and hence difficult to control.Molten salts were also used as reactants for a bottomup production of a few layered high-quality 2D materials. [57,58]ince molten salt is a versatile reaction medium, characterized by a wide range of salt types and melting temperatures, it can be tailored to optimize the exfoliation of 2D crystals into thin sheets.
The solubility of the source 2D materials in the molten salt plays a vital role in their processing. [59]Molten salt treatment of insoluble layered materials (e.g., graphite) resulted in exfoliation, where the impregnation of the molten salt ions into graphite interlayers introduced gentle shear forces that yielded large-sized graphene nanoplatelets. [3,60][67][68] We ask if the molten salt approach could also serve as 1) a medium for top-down exfoliation of insoluble 2D materials (e.g., graphite); or 2) a solvent for bottom-up growth of thin sheets of soluble substances, where we take MoS 2 as a test case.Developing methods for synthesizing MoS 2 with controllable size, shape, and edge structure is crucial for achieving tunable properties and optimizing device performances.Various parameters affect the composition, microstructure, and morphology of the generated 2D materials (e.g., reaction temperature, treatment time, MoS 2 :salts ratio, and more).However, as a test case, we focused on the molten salt treatment of high and low surface area MoS 2 (micron-size particles and a single large pellet, respectively) in various molten salt media (CsCl and NaCl/KCl).
Thus, we examined the balance between exfoliation and solubility of MoS 2 in various molten salt media.We found that molten CsCl salt treatment of MoS 2 (soluble) is surface areadependent and yielded faceted crystals or flowerlike structures via a dissolution-growth mechanism rather than exfoliation.The insight into the effect of molten salt on the properties of insoluble (graphite) and soluble (MoS 2 ) 2D materials provides an excellent platform for the preparation of various types of morphologies and tune their properties according to the required application.These include graphene-based nanomaterials for thermal energy storage [3,69] and MoS 2 thin sheets for photocatalysis and sensing. [70]Importantly, the developed approach could be applied to the exfoliation and recrystallization of other commercially available 3D layered materials to 2D materials.

Results and Discussion
The effect of low and high MoS 2 concentration in the molten salt was achieved by using surface-lean (SL) and surface-rich (SR) approaches, respectively (Figure 1).In an SL approach, a single large (9 mm) MoS 2 pellet (Figure 1a, Section 2.1) is used as a source material, and in the SR, multiple 5 μm MoS 2 particles (Figure 1b, Section 2.2).The structure and morphology of the products were fully characterized and compared to the initial MoS 2 crystalline particles.Finally, the effect of molten salt thermal treatment on soluble (MoS 2 ) and insoluble (graphite) layered materials is summarized (Section 2.3).

The SL Approach
After a thermal treatment of a single large MoS 2 pellet in molten salt, the pellet (visually intact, Figure 2a) was mechanically separated from the salt.The MoS 2 product in the salt resulted from recrystallization of the dissolved MoS 2 originating from the single pellet.
The MoS 2 concentration (measured by inductively coupled plasma optical emission spectrometry [ICP-OES]) in the CsCl salt was higher than in NaCl/KCl salt (0.033 AE 0.005 wt% versus 0.0013 AE 0.001 wt%, respectively), in agreement with the values  The SL approach: a1) a single large MoS 2 pellet was molten salttreated at 750 °C for 3 h in NaCl/KCl or CsCl salts.After treatment, the single pellet was mechanically removed from a2) the NaCl/KCl or a3) CsCl salts.b) The remaining CsCl sample after treatment was darker than that of c) the NaCl/KCl salt indicating higher MoS 2 solubility in CsCl. [71]cale bar = 1 cm.reported in the literature. [71]Visual inspection indicated that after the molten salt treatment, the CsCl salt was darker than NaCl/KCl salt (Figure 2b,c, both originally white).
The formation mechanism of MoS 2 flowers involves a dissolution-nucleation-recrystallization process, which is well known for various bottom-up approaches, namely, hydrothermal and solvothermal treatments (Table S2, Supporting Information), [26, but not for top-down processes.
The single MoS 2 pellet was partially dissolved in the salt (i.e., NaCl\KCl and CsCl) to form MoS 2 thin sheets (%5 nm in thick, Figure 4).99] Finally, the flowerlike structures of MoS 2 transform into welldefined MoS 2 micro-flowers (20 μm in size, Figure 3a) through the Ostwald ripening process. [26, The fwerlike morphology was kept in the salt-free MoS 2 , as shown by scanning electron microscope (SEM) and TEM imaging (Figure 5a-c inset, respectively).The high concentration of MoS 2 in CsCl (due to higher MoS 2 solubility [71] ) enables crystal growth and the formation of large flowers, while in NaCl/KCl, the low concentration restricts the growth and hence much smaller flowers were recorded.Therefore, the size of the flowers obtained via thermal treatment in CsCl (6.1 AE 3.5 μm in flower diameter; Figure 5b) was four times larger than those obtained in the NaCl/KCl system (1.4 AE 0.4 μm in flower diameter; Figure 5a).Furthermore, the number of flowers formed in CsCl (as measured by SEM) was higher than that in NaCl/KCl (surface density of 8.3 and 1.6 per 100 Â 100 μm 2 , respectively).This corresponds with the substantially higher MoS 2 solubility in CsCl than in NaCl/KCl, as discussed earlier.
Electron diffraction of the salt-free flowers (Figure 5c), obtained via molten CsCl treatment, demonstrated a powder pattern resulting from the polycrystalline assembled sheets, indexed to the (004) and ( 102) planes of MoS 2 characteristic. [100]imilar MoS 2 flower morphology was reported in several bottom-up syntheses in various media, [38,39,65] including molten salts, [57,58] where common features of these syntheses are the use of MoS 2 precursors and the formation of by-products that are difficult to remove.Moreover, the complicated chemical reactions sequence of these syntheses makes it difficult to monitor and scale up.Our study suggests that MoS 2 1) dissolved in molten salt medium from a single large pellet and formed MoS 2 crystalline thin-sheet through a top-down approach, which 2) aggregate to form MoS 2 flower, similar to those obtained by bottom-up MoS 2 .
Our results demonstrate that flowerlike MoS 2 assemblies are obtained in top-down process, directly from the as-received MoS 2 crystal, and not restricted to a bottom-up process, hence eliminating the need for Mo and S precursors and the potential byproduct or unreacted species.The much lower MoS 2 solubility in NaCl/KCl compared to CsCl resulted in much fewer and much smaller flowers (Figure 5), demonstrating the crucial role of MoS 2 solubility in the salt.

The SR Approach
We examined the effect of concentration of dissolved MoS 2 in the salt by manipulating the surface area of the source MoS 2 crystals (Figure 1).Similar thermal treatments in molten salts were applied on multiple small MoS 2 particles (instead of a single large pellet).The high surface area of the MoS 2 particles dictated high dissolved MoS 2 concentration in the salt (relative to the SL approach), assessed by the change in the average diameter of the MoS 2 particles (see Experimental Section).Molten salt treatment of the multiple MoS 2 particles yielded a black homogeneous mixture in both salts (Figure 6).The salt-free MoS 2 product was characterized, as discussed later.
The MoS 2 crystals obtained via treatment of as-received crystalline MoS 2 particles in molten NaCl/KCl were ragged, irregular particles with characteristic layered morphology (TEM, Figure 7b), similar to the as-received MoS 2 crystal (SEM, Figure 7a).In contrast, the product of the CsCl-treated MoS 2 consisted of faceted crystals (TEM, Figure 7c) in agreement with SEM imaging (Figure S2, Supporting Information).Both samples showed electron diffraction of 2 H-MoS 2 lattice structure (Figure 7c, left inset).In addition, the size of the particles obtained via treatment in CsCl and NaCl/KCl was smaller than that of the as-received MoS 2 sample (Figure 7, right insets).
The MoS 2 concentration in the CsCl was higher than in the NaCl/KCl salt (5.3 and 3.3 wt%, respectively), and about two orders of magnitude larger than the concentration obtained in the SL approach (Section 2.1).A possible explanation relates to the high surface area of the MoS 2 particles dictating high dissolved MoS 2 concentration in the salt (compared to the SL approach).
Several techniques have been used to demonstrate the absence of intercalation or chemical reaction between the salts (e.g., NaCl/KCl and CsCl) and MoS 2 , as described later.
Typical X-ray diffractograms of SR products from both CsCl and NaCl/KCl treatment before salt removal indicated a hexagonal phase of MoS 2 (Figure S3a, Supporting Information) with the salt reflections.When the salt was removed, these patterns were similar to that of the as-received MoS 2 crystals (Figure S3b, Supporting Information) with no additional peaks, attesting to efficient salt removal.
Further, X-ray photoelectron spectroscopy (XPS) of both faceted crystals (MoS 2 -CsCl-SR) and irregular particles (MoS 2 -NaCl/KCl-SR) products (after salt removal via washing) showed characteristics of the as-received MoS 2 crystals (Mo 3d 5/2 and Mo 3d 3/2 peaks at binding energy %229.7 and %232.8 eV, respectively, and S 2s peak at %227.1 eV, Figure S4, Supporting Information).These results rule out any doping or intercalation of the salt during the molten salt treatment of the MoS 2 particles.
Let us return to the crystal morphology.The higher MoS 2 solubility in CsCl (compared to NaCl/KCl) [71] dictated a change from ragged, irregular-to-faceted morphology (Figure 7c and S2c, Supporting Information), which could be related to a dissolution-nucleation-recrystallization process. [101]The total concentration of MoS 2 particles in the salt is 10 wt% (1:9 MoS 2 :salt w/w), much higher than the dissolved MoS 2 concentration in the CsCl salt using the SL (0.033 wt%) or SR (5.3 wt%) approaches.
The coexistence of MoS 2 particles with dissolved MoS 2 species leads to growth of hexagonally shaped faceted crystals, thermodynamically preferable over other structures, that is, thin sheets or irregular MoS 2 particles. [102,103]It entails the occurrence of a dissolution-nucleation-recrystallization process.First, the 5 μm MoS 2 particles partially dissolved in the molten CsCl.Then, the dissolved MoS 2 crystalize on the surface of an existing MoS 2 particle (via Ostwald ripening, [66,67] well-known for ceramics/molten salts systems [61] ) to faceted hexagonal crystal.
Interestingly, no flowerlike structures were detected in the SR process by SEM and TEM micrographs of the MoS 2 products (Figure S2 and S7, Supporting Information), as opposed to  the SL process for both CsCl and NaCl/KCl salts (Figure 5) where the dissolved MoS 2 coexisted only with a single large MoS 2 pellet of negligible surface area.Thus, the SR approach yields faceted hexagonal MoS 2 crystals rather than flowerlike products.It has been demonstrated that faceted MoS 2 crystals are often obtained via various preparation techniques of bulk or thin sheets; [45,55] however, as mentioned, these techniques are monitored by many parameters and hence difficult to control.Herein, we developed methods for synthesizing MoS 2 sheets with controllable size, shape, and edge structure by simple molten salt treatment of irregular-shaped MoS 2 particles. 06]

Guideline for Molten Salt Treatment of Layered Materials
When a layered material is insoluble in the molten salt (e.g., graphite), it exfoliates: [3,60] the molten salt impregnates the 2D galleries, and widens the interlayer spacing, which results in large-sized and high-quality 2D nanoplatelets.
When layered material is soluble in the molten salt (e.g., MoS 2 ), a dissolution-nucleation-recrystallization process results in surface area-dependent morphologies, namely, SR !faceted crystals and SL !flowerlike, as summarized in Table 1.
In the SL approach, the dissolved MoS 2 species coexisted with a single MoS 2 pellet of negligible surface area.Therefore, the MoS 2 product was attributed solely to the dissolved MoS 2 from the pellet, that upon cooling undergoes homogeneous crystallization, that is, nucleation-recrystallization.This process yielded flowerlike morphology consisting of thin sheets of MoS 2 forming flowerlike morphology, similarly to bottom-up synthesized MoS 2 . [38,39,57]n the SR approach, the dissolved MoS 2 coexisted with multiple small MoS 2 crystals of high surface area.Therefore, the dissolved MoS 2 product crystalized on the surface of the existing MoS 2 particles, that is, heterogeneous crystallization, to form hexagonally shaped faceted MoS 2 crystals rather than flowers.
Faceted 2D crystals have been explored for various applications, for example, catalysis and batteries, [107,108] due to their facet-dependent performances.In particular, 2D semiconductor materials with controllable facet orientation greatly influence the electronic structure and bandgap.Usually, different facets evolve after a complicated growth process, that is, CVD. [109]he faceted MoS 2 crystals were obtained by a simple molten salt treatment without surfactants or solvents. [110,111]The process is simple to control by temperature and treatment time.Therefore, precise tailoring of parameters in the molten salt process is imperative to the formation of facet-controlled 2D materials and the exploration of facet-dependent properties and applications.
Overall, the molten salt treatment approach offers versatility in producing exfoliated/thin sheets or faceted crystals of 2D-layered materials, that is, morphology control, by choosing an appropriate salt.In this study, as a proof of concept, we prepared 2D materials with controllable size and shape from MoS 2 (soluble in salt) or graphite (non-soluble in salt) in a simple and affordable way.

Conclusions
This study examined the balance between exfoliation and solubility of MoS 2 in various molten salt media.Molten CsCl treatment of multiple-micron-size MoS 2 particles (SR) resulted in a substantial change in morphology (irregular !faceted crystals, SEM, and TEM) through a dissolution-recrystallization process.However, the irregular morphology of the MoS 2 crystals was not changed after molten NaCl/KCl treatment, in which MoS 2 is less soluble.The molten salt treatment did not affect the crystallinity, d-spacing, and purity of the final products (TEM, X-ray diffraction [XRD], and XPS).Further, the experiment with a single MoS 2 pellet (SL) yielded a flowerlike structure.In contrast, thermal treatment of an insoluble 2D material (e.g., graphite) resulted in exfoliation.
The molten salt treatment offers a simple and scalable approach for producing exfoliated thin sheets or faceted crystals of 2D-layered materials from graphite and MoS 2 .As such, we expect to extend it to various soluble (TMD) and non-soluble (boron nitride) materials.
Thermal Treatment of MoS 2 in Molten Salts: 2D MoS 2 was prepared using MoS 2 SL and SR approaches (Figure 1 and S5, Supporting Information).
In the SL approach, a large single MoS 2 pellet (0.5 g pressed at 20 bar in a hydraulic press, APEX, A-14, UK, diameter and thickness of 9 and 3 mm, respectively) and salt (either a eutectic mixture of NaCl and KCl [1:1 mol ratio] or CsCl), 1:9 MoS 2 :salt w/w, were placed in an alumina crucible and inserted into a vertical tube reactor equipped with a temperature controller (Figure 1a).The reactor was heated (10 °C min À1 ) under argon flow (50 mL min À1 , to avoid oxidation) up to 750 °C (above the melting point of the salts, 657 °C for NaCl/KCl and 646 °C for CsCl). [46]The mixture was then held at 750 °C for a 3 h thermal treatment during which, the single MoS 2 pellet was partially dissolved in the salt.After natural cooling (%10 °C min À1 ), the partially dissolved MoS 2 pellet was removed and the MoS 2 product was extracted from the remaining solid phase (gray Table 1.Molten salt treatment of layered materials is solubility dependent.Insoluble 2D (e.g., graphite) exfoliates, [3,60] and soluble (e.g., MoS 2 ) undergo surface-dependent dissolution-nucleation-recrystallization process, namely, surface rich !faceted crystals, and surface lean !flower like. in Figure 1a) by removing the salt: washing with 200 mL warm water (80 °C) and vacuum filtration (Sartorius paper, 0.2 μm pores).The filtered, salt-free MoS 2 product was dried at 80 °C for 24 h.In the SR approach, 0.5 g of small MoS 2 particles (%5 μm, Figure 1b) were mixed with salt (either NaCl/KCl [1:1] or CsCl) in a mortar to form a homogeneous mixture (Figure 1b).This mixture was thermally treated in the same way as in the SL approach.
The salt-free products obtained via the SL approach (thermal treatment of a single MoS 2 pellet) in NaCl/KCl or CsCl were termed MoS 2 -NaCl/ KCl-SL or MoS 2 -CsCl-SL, respectively.The salt-free products obtained via SR approach (thermal treatment of multiple particles) were termed MoS 2 -NaCl/KCl-SR and MoS 2 -CsCl-SR, respectively.
Characterization of MoS 2 Products: XRD of the samples was recorded using a PANalytical Empyrean Powder Diffractometer (Cu Kα1 radiation, step size of 0.02°(2θ) in a 2θ range of 5°-55°).
SEM (Thermo Fisher, Verios 460L) equipped with an EDS detector (Oxford Instruments) was used for imaging.The powder samples were secured on an aluminum stub using conductive carbon tape before loading into the SEM chamber.
TEM of the samples was performed on Thermo Fisher Tecnai 12 G2 TWIN and JEM 2100 F microscopes operating with an accelerating voltage of 120 and 200 kV, respectively.Samples were dispersed and sonicated (5 min) in ethanol before drop-casting on a lacey carbon-coated copper grid (300 mesh, Lacey carbon support films, Ted Pella) and dried in ambient conditions.
Raman spectrometry (Horiba Jobin Yvon HR LabRAM micro-Raman) was used to measure powder samples (placed on a glass slide, excitation laser wavelength of 532 nm with a grating of 1800 grooves mm À1 ).
XPS measurements were performed with an ESCALAB 250 (10 À9 mbar, Al Kα X-ray source).The samples were pressed into an indium foil prior to the measurement.
ICP-OES was carried out on SPECTRO ARCOS ICP-OES analyzer to measure the MoS 2 content in the salt after treatment of MoS 2 pellet in NaCl/KCl or CsCl and its mechanical separation (SL approach).The salt samples, typically 50 mg, were dissolved in 70% concentrated nitric acid.In the SR approach, the dissolved MoS 2 concentration in the salt couldn't be measured by ICP-OES since the MoS 2 particles and the dissolved MoS 2 were indistinguishable (micron-size particles), hence the solubility was assessed by average diameter change of the MoS 2 particles.
AFM was performed by a Dimension 3100 SPM instrument, operated in a tapping mode with Veeco RTESP silicon tips to measure the MoS 2 thickness.The sample was prepared by placing a droplet of dispersed MoS 2 in ethanol on a SiO 2 wafer and allowing it to dry on a hot plate for several minutes before the measurements.

Figure 1 .
Figure 1.Schematic illustration (not to scale) of a) the surface-lean (SL) approach-a single large MoS 2 pellet was mixed with salt and dissolute during the thermal treatment, resulting in flower crystals; b) the surfacerich (SR) approach-MoS 2 particles were mixed with salt.The dissolved MoS 2 product crystalized on the surface of the existing MoS 2 particles to form either faceted or ragged, irregular crystals (CsCl and NaCl\KCl, respectively).

Figure 2 .
Figure2.The SL approach: a1) a single large MoS 2 pellet was molten salttreated at 750 °C for 3 h in NaCl/KCl or CsCl salts.After treatment, the single pellet was mechanically removed from a2) the NaCl/KCl or a3) CsCl salts.b) The remaining CsCl sample after treatment was darker than that of c) the NaCl/KCl salt indicating higher MoS 2 solubility in CsCl.[71]Scale bar = 1 cm.

Figure 3 .
Figure 3. a) Scanning electron microscope (SEM) micrograph and b,c) energy-dispersive spectrometer (EDS) of MoS 2 -salt mixture (before salt removal via washing) obtained via molten treatment in CsCl of a MoS 2 pellet (SL).The EDS maps (Mo-blue (b) and S-pink (c)) indicate the composition of the flowerlike structures.Scale bar = 10 μm.

Figure 6 .
Figure 6.SR: products before salts washing obtained after molten salt treatment of the MoS 2 particles at 750 °C for 3 h in molten a) NaCl/KCl and b) CsCl.

Figure 7 .
Figure 7. a) SEM micrograph of as-received MoS 2 and TEM micrographs of MoS 2 product obtained via treatment in molten b) NaCl/KCl and c) CsCl.The irregular edges of the as-received MoS 2 crystal and MoS 2 -NaCl/KCl-SR became faceted after treatment in molten CsCl, indicating a dissolution-nucleation-recrystallization process.Left inset: diffraction ([001] zone axis) showing the structure of the faceted MoS 2 .Right inset: MLD histograms extracted from SEM (n = 100 particles).Scale bar = 0.5 μm.