Stabilization of 1T-MoS2 Sheets by Imidazolium Molecules in Self

Jul 30, 2015 - The 1T-to-2H conversion of MoS2 can be revealed by DSC through registration of a broad exothermic peak, reflecting restoration of the m...
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Stabilization of 1T-MoS2 Sheets by Imidazolium Molecules in SelfAssembling Hetero-layered Nanocrystals Alexander S. Goloveshkin,† Ivan S. Bushmarinov,† Alexander A. Korlyukov,† Mikhail I. Buzin,† Vladimir I. Zaikovskii,‡,§ Natalia D. Lenenko,† and Alexandre S. Golub*,† †

A.N. Nesmeyanov Institute of Organoelement Compounds, Russian Academy of Sciences, Vavilova St. 28, 119991 Moscow, Russia Boreskov Institute of Catalysis, Siberian Branch of Russian Academy of Sciences, Lavrentieva Ave. 5, 630090 Novosibirsk, Russia § Novosibirsk State University, Pirogova St. 2, 630090 Novosibirsk, Russia ‡

S Supporting Information *

ABSTRACT: We report a facile, room-temperature assembly of MoS2-based hetero-layered nanocrystals (NCs) containing embedded monolayers of imidazolium (Im), 1-butyl-3-methylimidazolium (BuMeIm), 2-phenylimidazolium, and 2-methylbenzimidazolium molecules. The NCs are readily formed in water solutions by selforganization of the negatively charged, chemically exfoliated 0.6 nm thick MoS2 sheets and corresponding cationic imidazole moieties. As evidenced by transmission electron microscopy, the obtained NCs are anisotropic in shape, with thickness varying in the range 5−20 nm and lateral dimensions of hundreds of nanometers. The NCs exhibit almost turbostratic stacking of the MoS2 sheets, though the local order is preserved in the orientation of the imidazolium molecules with respect to the sulfide sheets. The atomic structure of NCs with BuMeIm molecules was solved from powder X-ray diffraction data assisted by density functional theory calculations. The performed studies evidenced that the MoS2 sheets of the NCs are of the nonconventional 1T-MoS2 (metallically conducting) structure. The sheets’ puckered outer surface is formed by the S atoms and the positioning of the BuMeIm molecules follows the sheet nanorelief. According to thermal analysis data, the presence of the BuMeIm cations significantly increases the stability of the 1T-MoS2 modification and raises the temperature for its transition to the conventional 2H-MoS2 (semiconductive) counterpart by ∼70 °C as compared to pure 1T-MoS2 (∼100 °C). The stabilizing interaction energy between inorganic and organic layers was estimated as 21.7 kcal/mol from the calculated electron density distribution. The results suggest a potential for the design of few-layer electronic devices exploiting the charge transport properties of monolayer thin MoS2. metallic,19−21 thus allowing, for instance, realization of electronic hetero-structures within the parent structure17 and preparation of highly conductive films,22 supercapacitor electrode materials,23 and efficient catalysts for the hydrogen evolution reaction (HER).24,25 The electron doping of the MoS2 sheets provided by the electric double layer, in which the induced surface negative charge is balanced by the imidazolium or alkylammonium cations of the ionic liquid salt, was recently shown to result in metallic behavior of these sheets, providing excellent band transport characteristics in active field-effect transistor channels2,26,27 and even superconducting properties.28 The electron doping leading to the 2H-to-1T structural phase transition can also be provided by the hot electrons generated by plasmonic metal nanoparticles deposited on a MoS2 monolayer, resulting in enhancement of the MoS2 HER activity.29,30

1. INTRODUCTION Due to enhanced charge-carrier and semiconducting properties of molybdenum disulfide,1,2 materials based on its mono- and few-sheet forms3−5 are attracting increasing attention as possible components of future electronic and optoelectronic devices6,7 and photocatalysts.8 Many of the applications imply participation of MoS2 sheets in charge-transfer processes as electron acceptors. Negative charge transfer of more than 0.1 e per Mo is known to induce structural rearrangement in the parent MoS2 atomic structure,9 with full transformation expected at >0.4 e per Mo according to recent quantum chemical calculations.10,11 It occurs as a transition from the ambient-stable 2H-MoS2 modification, containing Mo atoms in a trigonal prismatic surrounding of S atoms, to the so-called 1T modification,12 characterized by an octahedral Mo−S coordination environment and the presence of zigzag (Mo)n chains.13−17 The 2H-to1T transformation was recently found to involve gliding atomic planes of sulfur and/or molybdenum, and it proceeds via intermediate phases.18 Such a transition was shown to switch the properties of the MoS2 sheet from semiconducting to © 2015 American Chemical Society

Received: June 26, 2015 Revised: July 30, 2015 Published: July 30, 2015 8953

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2.2. Methods. High-resolution (HR) TEM images were obtained on a JEM-2010 electron microscope (JEOL Ltd.) with a lattice-fringe resolution of 0.14 nm at an accelerating voltage of 200 kV. The HRTEM images of periodic structures were analyzed and filtered by the fast Fourier transformation (FFT) method with the help of computer software, Digital Micrograph 3.6.5 (Gatan Inc.). Particles to be examined by TEM were deposited on a perforated carbon film mounted on a copper grid. The powder diffraction patterns of the NCs were measured using a Bruker D8 Advance Vario diffractometer equipped with a Ge(111) Cu Kα1 monochromator and a LynxEye 1D silicon strip detector, in transmission between Kapton films. The measurement range was 6− 120° 2θ, and the step size was 0.00917° 2θ. All modeling and indexing was performed using TOPAS 4.2 software.37 The details of the structure refinement procedure are described in the Supporting Information. DSC measurements were done using a Mettler DSC-822e differential scanning calorimeter at a heating rate of 10 °C/min in an argon atmosphere. TGA was performed on a Derivatograth-C instrument (Hungarian Optical Works, Budapest) under argon flow at a heating rate of 10 °C/ min. 2.3. Quantum-Chemical Calculations. DFT (PW-PBE-D) periodic calculations were performed in the VASP package.38−41 Projector augmented wave (PAW) pseudopotentials42,43 were used for all atoms. Exchange and correlation terms of total energy were described by a PBE functional,44 with Grimme D2 van der Waals correction.45 Topological analysis was carried out by using the AIM program.46 Other details of DFT calculations and quantum theory of atoms in molecules (QTAIM) analysis are presented in the Supporting Information.

Metallic 1T-MoS2 modification is known to be stable only in the presence of excess negative charge on the MoS2 sheets, which is usually provided by the electrons from intercalated alkali metals (M = Li, Na) in M+x(MoS2)x− compounds.31,32 Such derivatives are extremely air-sensitive, whereas, a “neutral” 1T-MoS2 resulting from removal of Li or Na tends to revert to 2H (semiconductive) form on gentle heating (∼100 °C)12 or even on aging at room temperature.33,34 Safe, convenient, and “tunable” means for stabilizing the metallic MoS2 state in 2D elements of the devices remains a challenge. It appears that fixation of a sufficient amount of organic cations on the sulfide sheet surface to balance the excess negative charge of the sheets may provide the required stabilization effect. It was already shown that a significant amount of alkylammonium cations (as many as ∼0.3 mol/mol of MoS2) can be trapped between the MoS2 sheets in few-layer nanocrystals (NCs) obtained through self-assembly of the negatively charged sheets, resulting from chemical exfoliation of LiMoS2, and the corresponding cationic species.35 We hypothesized that layered NCs of this type with imidazolium cationic molecules may also be stable. In this case they could serve as excellent model compounds for undestanding the charge-transfer-induced rearrangements occurring in their hetero-structures with MoS2 sheets and, possibly, as 2D units with stabilized 1T metallic MoS2 modification for few-layer electronic devices. The objective of this work was to study the self-assembly of novel MoS2-based nano-hetero-structures with imidazolium molecules, including ionic liquid ones, and to reveal the structure and morphology of the MoS2 sheets as well as the geometry and energy of their bonding with imidazolium derivatives. To elucidate these details, we employ powder X-ray diffraction (PXRD), utilizing an original, recently developed approach for turbostratically disordered MoS2-based solids,36 transmission electron microscopy (TEM), thermogravimetric analysis (TGA) and differential scanning calorimetry (DSC) measurements, and density functional theory (DFT) calculations.

3. RESULTS AND DISCUSSION 3.1. Assembly Process of Nanocrystals. The negatively charged (MoS 2 ) x− sheets were produced by chemical exfoliation of LiMoS2 in aqueous media, which is known to lead to the monolayer thin MoS2.3,13,17,47 These sheets were then used to assemble NCs with cationic imidazole derivatives taken in a reaction as a salt (BuMeIm) or obtained in situ by protonation of neutral imidazole molecules. A schematic of the synthesis is shown in Figure 1. In all reaction mixtures, the

2. EXPERIMENTAL SECTION 2.1. Materials Synthesis. The NCs were assembled in solutions from the negatively charged molybdenum disulfide [(MoS2)x−] sheets and cationic imidazole derivatives. Initially, purified natural molybdenum disulfide (DM-1, Scopin Factory, Russia) with a particle size (95%) smaller than 7 μm was intercalated with lithium by treating it with an excess of 1.6 M n-butyllithium solution in hexane (Aldrich) for 1 week, followed by washing with hexane to remove unreacted nbutyllithium and vacuum drying. The obtained crystalline product, Li1MoS2, was placed in bi-distilled water, sonicated for 15 min, and then stirred on a magnetic stirrer for 30 min to prepare the 1 mg mL−1 aqueous dispersion of MoS2 containing the exfoliated, negatively charged molybdenum disulfide [(MoS2)x−] sheets. Solutions of the chlorides of imidazolium (Im), 1-butyl-3-methylimidazolium (BuMeIm), 2-phenylimidazolium (PhIm), and 2-methyl-benzimidazolium (MeBenzIm) were prepared by dissolving the corresponding salt (1butyl-3-methylimidazolium chloride, Aldrich) or neutral base (imidazole, 2-phenylimidazole, or 2-methylbenzimidazole, all purchased from Aldrich) in water, which was acidified with HCl in the case of neutral bases. Next, 20 mL of the solution containing the imidazolium derivative in an amount of 10 mol/mol of MoS2 was added to 200 mL of the MoS2 dispersion, and the mixture was stirred on a magnetic stirrer for 1 h. The formed NCs were isolated by centrifugation, washed three times with water, and dried in vacuum. The composition of the products was determined from elemental analysis data (C, H, N, Mo).

Figure 1. Assembly of the MoS2 sheets and imidazolium cations into the nanocrystals.

components readily participated in the assembly process, yielding flake-like particles which settled at the bottom of the reaction vessel. Low-magnification TEM showed that they are of lamellar shape with differing lateral sizes (typically, hundreds of nanometers) and exhibit evident flexibility and a tendency to 8954

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Langmuir bending when being deposited on a microscope grid (see Figure 2a,b). According to the elemental analysis data, the imidazolium moieties are contained in these substances in amounts of more than 0.1 mol/mol of Mo (Table 1). The NCs with BuMeIm, Table 1. Synthesis Conditions, Composition, and Interlayer Distances of MoS2-Based Nanocrystals imidazole derivative

pH

Im:MoS2 (mol/mol)

interlayer distance, c (Å)

organic layer thickness, Δc (Å)

Im BuMeIm PhIm MeBenzIm

2.9 10.6 2.4 2.0

0.13 0.11 0.11

9.7 10.0 10.7 9.8

3.5 3.8 4.5 3.5

MeBenzIm, and PhIm showed good stability, keeping their elemental and phase composition unchanged during at least several months, while unsubstituted Im molecules progressively left the structure, in particular upon vacuum drying. The escape of unsubstituted Im presumably occurs because of the partial deprotonation of inserted cations on aging and the rather high volatility of the resulting neutral imidazole molecules. Notice that MeBenzIm and PhIm molecules can also be sensitive to deprotonation; however, easy removal of the deprotonated form favors this process in the case of Im. Interlayer lattice expansion of MoS2 with respect to the pristine 2H-MoS2 (Δc), measured from the positions of the 00l peaks in PXRD, allowed a rough estimation of the thickness of the organic layer composed of the corresponding imidazolium molecules. The greatest Δc value (∼4.5 Å) was observed in the case of PhIm, which contains a bulky ring substituent rotated relative to the imidazolium ring.48 For other compounds, this value is less by ∼1 Å and approaches the van der Waals thickness of flat aromatics (∼3.5 Å). One could suppose that the orientation of Im, BuMeIm, and MeBenzIm within the organic layer is strictly parallel to the sulfide sheets, but closer inspection of the NC structure complicates this model, as is shown below. 3.2. High-Resolution Transmission Electron Microscopy Study. A HRTEM study performed on the crystals with MeBenzIm and BuMeIm confirmed that self-organization of the components results in few-sheet NCs. Their thickness, measured on the side projections, typically varies in the range 5−20 nm (Figure 2 and Supporting Information Figure S1). The hetero-layered character of the NCs is visible from the presence of weak-contrast strips between the adjacent MoS2 strips of high contrast, in agreement with previous observations for layered MoS2-based compounds. Rather regular alternation of the hetero-layers is consistent with a good agreement between the mean interlayer distances determined by XRD and those measured locally by FFT or periodic contrast profile image processing (Figures 2 and S1), or by analysis of the individual distance distribution histograms (Figure S2). This alternation distinguishes the present layered compounds from the composite structures obtained by surface functionalization of the MoS2 layers with organic species, which does not imply regular layer stacking.49−51 While the NCs are relatively ordered in terms of the thickness of alternating hetero-layers, their stacking exhibits disorder evident in the frontal projections due to the appearance of the nanosized domains differing in crystallographic orientations (Figure S3).

Figure 2. (a,b) Low- and (c) high-resolution TEM images of the nanocrystals with BuMeIm (a,c) and MeBenzIm (b). FFT image of the framed region and periodic contrast profile are shown in the insets.

HRTEM images also evidence the difference between the MoS2 sheet structure in the present compound and in the initial 2H-MoS2. Figure 3 shows the side projection of a perfectly

Figure 3. HRTEM image of BuMeIm-MoS2 and FFT image of the framed region, with periodicities values given for different directions.

ordered NC fragment of BuMeIm-MoS2 and its FFT image with periodicities measured in two directions. Along the layers, both the basic (d1 0 0 ≈ 0.275 nm) and superstructure (d1/2 0 0 ≈ 0.55 nm) periodicities relative to the initial hexagonal unit cell are visible. The superstructure ordering, with approximately double the basic periodicity, was previously observed in the sheets of chemically exfoliated MoS2 and interpreted as a consequence of the periodic in-layer displacement of the Mo atoms caused by charge transfer.13−17 Thus, HRTEM results strongly suggest that, at least locally, the MoS2 sheet structure in the obtained NCs corresponds to 1T modification. 3.3. Nanocrystal Structure Determination. Further insight into the structure of the NCs was obtained by modeling the PXRD pattern of the most ordered compound, 8955

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MoS2 layers, caused most likely by their interactions with organic cations in the NCs.

(BuMeIm)0.13MoS2. As can be seen in Figure 4, the general view of the pattern is typical of MoS2 layered compounds. It

Table 2. Selected Characteristics of the BuMeIm-MoS2 Nanocrystal Structurea parameter Mo−Mo (Å)

Mo−Mo−Mo (deg) Mo−S (Å) interlayer shift along a axis (Å) along b axis (Å) height variation in S positions on the sheet surface (Å)

refined

calculated

2.834(4) 3.209(6) 3.727(6) 68.96(14) 2.398(6)

2.804 3.209 3.762 69.73 2.440

0.68(1) 0.744(6) 0.455(3)

0.68 0.744 0.364

Figure 4. Powder X-ray diffraction pattern of (BuMeIm)0.13MoS2 and its fit. Only the main reflection positions are labeled.

a

contains strong, relatively sharp 00l reflections and broad, asymmetric hk0 ones, thus pointing to turbostratic disorder in the crystal structure. It is generally known that turbostratic character of the layer stacking decreases dramatically the possibilities for estimating the exact atomic structure from PXRD data by commonly used protocols. Owing to the random rotations and shifts of the layers, hkl diffraction peaks almost disappear; the others broaden and overlap, making a full profile description of such a patternnecessary for structural model refinementvery difficult to provide. However, Ufer’s “supercell approach” 52 has been successfully applied to structural analysis of the systems involving turbostratic clays, and we have recently tailored this method to study the MoS2−Et4N+ layered system, for which a structure agreeing well with EXAFS data and periodic DFT calculations has been obtained.36 The supercell approach exploits the similarity in diffraction patterns from single layers and turbostratic systems by describing a pattern with a virtual unit cell consisting of one layer of the refined structure and N > 10 empty layers. Combined with hkl-dependent broadening and scaling, this virtual structure reproduces the powder pattern well enough for direct Rietveld refinement of atomic positions and therefore structure determination. We used this method to solve and refine the structure of the (BuMeIm)0.13MoS2 NCs (see the Supporting Information for refinement details). The supercell approach requires an appropriate starting model. Taking into account that the MoS2 sheets are affected by the charge transfer, their starting geometry was assumed to be similar to the distorted octahedral geometry obtained for (Et4N)0.16MoS2 composition,36 in which the content of organic cations (0.16 cation/Mo) was close to that in the studied NCs (0.13 cation/Mo). Both the MoS2 inorganic layer structure and positions of imidazolium cations, including the rotation of the butyl substituent relative to the aromatic ring, were then refined. A single-layer model, implying that the individual “sheets” forming the turbostratically disordered system consist of exactly one inorganic layer and one organic layer, did not provide a satisfactory description of the powder pattern (Figure S4). We found, however, that a two-layer model, in which the hybrid MoS2−organic layers were allowed to shift relative to each other, results in a nearly perfect fit (Figure 4, Table 2). This indicates strong short-range correlation in the positions of

The refined geometry of the MoS2 sheets remained largely similar to the starting one, with the intralayer ordering described by a 5.700(10) Å × 3.209(6) Å superlattice (≈ √3a0 × a0, where a0 is a lattice parameter of the hexagonal unit cell of 2H modification), the octahedrally coordinated molybdenum atoms forming endless zigzag Mo−Mo chains along the b axis, and the sulfur atoms displaced in the c direction to form ridges and valleys alternating in the a direction on the MoS2 sheet surface (Figure 5). Therefore, the 1T-MoS2 modification is preserved in the sulfide sheets in the presence of imidazolium counterions. It is noteworthy that, in the studied NCs, this modification dominates in the molybdenum disulfide sheet structure, unlike the reported MoS2 nanosheets, not stabilized by organic cations, in which 1T-MoS2 only occupies some domains and coexists with comparable amounts of the domains of 2H modification.17,18,47,53 In the refined model of the NCs, the BuMeIm cations lie on the sulfide surface along the valleys, with the methyl and butyl substituents being embedded into the vallleys (Figure 5). The carbon atoms of the butyl group lie in the plane approximately parallel to the 001 crystal plane, i.e., parallel to the MoS2 sheets; the corresponding angle is as small as 3.8°. The imidazolium ring, however, is rotated noticeably with respect to both the 001 plane (by 19.3°) and the butyl fragment (24.4°). It is worth noting that the observed rotation of the butyl substituent relative to the aromatic ring (Figures 5 and S5) is noticeably smaller than one typically observed in molecular crystals with BuMeIm derivatives (45−130°), according to MOGUL analysis54 of the Cambridge Structural Database.55 This strongly suggests that the molecular conformation found in the NCs results from bonding interactions of imidazolium cations with a puckered MoS2 sheet surface. 3.4. Quantum-Chemical Calculations. To verify the structural model and to reveal the interactions between inorganic and organic components of the NCs with BuMeIm, periodic DFT calculations were carried out using the plane wave (PW) basis set and PBE functional (see Experimental Section). To fulfill the requirement of strictly ordered structure for periodic calculations, we constructed a hypothetical 3D model of the NC, in which the turbostratic disorder in sheet stacking was ignored. Imidazolium molecules were placed at the positions obtained in structure refinement with a supercell,

Full crystal structure data are presented in the Supporting Information (Tables S1−S7).

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0.13 with high accuracy. Importantly, the model placement of imidazolium cations avoids any unreasonably short cation− cation contacts. DFT calculations confirmed the structure determination results, giving parameters well matching the refined ones (Table 2). Study of the calculated structure in the framework of Bader’s quantum theory (QTAIM)56 was further performed. Topological analysis of the total electron density distribution revealed 28 (3,−1) bond critical points (BCPs) in the total electron density function ρ(r), reflecting the weak bonding interactions of the MoS2 sheets with imidazolium cations. We quantified the cation−sulfide sheet interactions using a correlation of the local kinetic energy in the (3,−1) critical point with the binding energy of an intermolecular contact (Econt), primarily developed by Espinosa et al. for weak hydrogen bonds57−59 and successfully applied for many kinds of other interactions,60 including those in the crystalline MoS2.61 The data extracted from the BCPs are summarized in Tables 3 and S8. Table 3. Contributions to the Total Energy of MoS2···Cation Dispersion Interactions contacts

Econt (kcal/mol)

no. of contacts

cation···MoS2 H(Bu)···S H(Me)···S H(Im)···S π···S

21.7 13.6 3.1 1.8 3.2

28 16 4 3 5

Considering the obtained binding energy values, one can see that CH···S interactions provide the main contribution (18.5 kcal/mol) to the overall cation binding with the MoS2 sheet surface (21.7 kcal/mol). The majority of these contacts belong to the butyl substituent, and their Econt constitutes the greatest part (63%) of the total binding energy. A significantly smaller contribution (3.2 kcal/mol) is provided by π−p interactions, i.e., the interactions between the C atoms of the imidazolium aromatic ring and the S atoms of MoS2 (Figure 5). The nature of some bonding contacts is illustrated in Figure 6 by mapping the laplacian of electron density function, −∇2ρ(r), the maxima of which show the regions of valence shell charge concentration (i.e., bonds and lone electron pairs). The relationship found between the binding energies of different molecule parts explains perfectly well the conformation adopted by the BuMeIm molecules in the assembled NCs, in particular the above-mentioned preferred orientation of the butyl and methyl fragments relative to the sheet relief. 3.5. Thermal Analysis. We expected that negative charge fixed on the sulfide sheets due to the presence of imidazolium cationic molecules in the NCs and noticeable bonding interactions between the sheets and BuMelm molecules revealed in our study should affect the thermal stability of the 1T-MoS2 structure in the NCs. To probe the phase transformations, we studied the thermal behavior of the NCs with BuMeIm. A thermal gravimetry (TG) curve of the NCs shows one-step weight loss in the interval of ∼160−370 °C (Figure S7a). Comparison of this curve with TG curves of pure 1-methyl-3-butyl-imidazolium chloride (Figure S7b) and pure 1T-MoS2 (Figure S7c) makes it evident that the weight loss of the NCs can be explained by the loss of the organic compound, though this process spans a broader temperature interval for the NCs as compared to the salt (∼220−340 °C).

Figure 5. Refined geometry of (a) MoS2 sheets and (b,c) BuMeIm molecules in the nanocrystals. Hydrogen atoms are omitted. The S atoms shifted outward (peach) and inward (light blue) in the sheet nanorelief are marked in (a), and the corresponding “ridges” and “valleys” are colored in (b). Bonding C···S (red) and CH···S (black) contacts are labeled in (c).

regularly distributed on the sulfide sheet (Figure S6) according to the ratio of 1 molecule per 8 Mo (BuMeIm0.125MoS2), which reproduces the experimental composition MeBuIm/MoS2 = 8957

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Figure 6. Plots of the laplacian of the electron density drawn in (a) S4−H3−C3, (b) S3−C6−H10, and (c) S3−S4−C2 planes. The atoms are labeled according to Figure 5. Isovalues are imaged on the arctangent scale, and their negative (blue solid line) and positive (red dashed line) values are shown. The borders of the atomic basins are depicted by bold solid lines, and the bonding interactions are marked by bold dashed lines.

their stacking is essentially turbostratic in nature. The crystal structure of these NCs was successfully determined at the atomic scale due to a combination of a new method for PXRD pattern analysis with DFT calculations and TEM results. This allowed us to reveal how the inorganic and organic components of the NCs adjust structurally to each other. With inclusion of imidazolium cationic molecules, the 1TMoS2-type structure with octahedral Mo−S coordination, nonequivalent Mo−Mo distances, and uneven surface was proven to be stabilized in the negatively charged sulfide sheets of the assembled NCs. The essential bonding interactions of the sulfide sheets with aliphatic and aromatic fragments of imidazolium molecules were evidenced in the NCs by QTAIM analysis of electron density distribution obtained by periodic DFT calculations. These interactions govern the positioning and conformation of the embedded imidazolium molecules. They also have an important influence on the overall stability of the 1T-MoS2 structure, resulting in a significant increase in the temperature of its phase transformation, as follows from DSC results. We believe that the results presented here will help in the design of new MoS2-based devices.

The 1T-to-2H conversion of MoS2 can be revealed by DSC through registration of a broad exothermic peak, reflecting restoration of the more stable MoS2 polymorph with trigonal prismatic polyhedra.12,19 For pure 1T-MoS2 obtained by assembling (restacking) of the MoS2 sheets in the absence of imidazolium cations, the irreversible transition takes place at ∼100 °C (Figure 7), consistent with previous observa-

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Figure 7. DSC curves for pure 1T-MoS2 (1,2) and BuMeIm-MoS2 nanocrystals (3,4) at first (1,3) and second (2,4) heating.

ASSOCIATED CONTENT

S Supporting Information *

tions.12,19,34 In case of the NCs, however, the transition is shifted to higher temperature (∼170 °C), which proves the stabilizing effect of BuMeIm cations on the 1T structure. In fact, 1T structure is preserved in the NCs up to the beginning of the removal of BuMeIm (Figure 7). It should be noted that an increase of the 1T-to-2H transition temperature was previously reported for MoS2 intercalates with polypyrrole62 and tetraazamacrocycles.63 As opposed to the previous works, our study allowed for identifying a true reason for such an effect via analysis of bonding interactions in the NCs, thus indicating a way for further tuning of this characteristic. Still, the overall stabilization effect is most likely kinetic in nature, since quantum-chemical calculations suggest that a significantly larger charge is required to keep the octahedral form of MoS2 thermodynamically stable.10,11

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.langmuir.5b02344. Additional TEM data, structure refinement details, refined NC structure characteristics, periodic DFT calculations and QTAIM analysis details, and TGA results (PDF) X-ray crystallographic data (CIF)

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Tel: +7-499-135-9376. Fax: +7499-135-9350.

4. CONCLUSIONS Negatively charged MoS2 sheets and imidazolium cationic molecules of different structures, brought in contact in water solution at room temperature, tend to self-organize into 5−20 nm thick 2D layered NCs with alternating inorganic and organic monolayers due to quenching of the MoS2 negative charge by the imidazolium cations. Though alternation of the monolayers in the NCs is rather regular in terms of thickness,

Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was financially supported by Russian Foundation for Basic Research (RFBR) grants 14-03-00287-a and 13-0312197-ofi-m. 8958

DOI: 10.1021/acs.langmuir.5b02344 Langmuir 2015, 31, 8953−8960

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DOI: 10.1021/acs.langmuir.5b02344 Langmuir 2015, 31, 8953−8960