Electrostatic Origin of Stabilization in MoS2–Organic Nanocrystals

Letter pubs.acs.org/JPCL

Electrostatic Origin of Stabilization in MoS2−Organic Nanocrystals Ivan S. Bushmarinov,*,† Alexander S. Goloveshkin,† Natalia D. Lenenko,† Vladimir I. Zaikovskii,‡,§ Alexander A. Korlyukov,† Alexandre S. Golub,*,† and Igor L. Eremenko†,⊥ †

A.N. Nesmeyanov Institute of Organoelement Compounds, Russian Academy of Sciences, Vavilova Street 28, 119991 Moscow, Russia ‡ Boreskov Institute of Catalysis, Siberian Branch of Russian Academy of Sciences, Lavrentieva Avenue 5, 630090 Novosibirsk, Russia § Novosibirsk State University, Pirogova Street 2, 630090 Novosibirsk, Russia ⊥ N.S. Kurnakov Institute of General and Inorganic Chemistry, Leninskii Prospekt 31, 119991 Moscow, Russia S Supporting Information *

ABSTRACT: Negatively charged molybdenum disulfide layers form stable organic− inorganic layered nanocrystals when reacted with organic cations in solution. The reasons why this self-assembly process leads to a single-phase compound with a welldefined interlayer distance in given conditions are, however, poorly understood to date. Here, for the first time, we quantify the interactions determining the cation packing and stability of the MoS2−organic nanocrystals and find that the main contribution arises from Coulomb forces. The study was performed on the series of new layered compounds of MoS2 with naphthalene derivatives, forming several distinct phases depending on reaction conditions. Starting with structural models derived from powder X-ray diffraction data and TEM, we evaluate their cohesion energy by modeling layer separation with periodic PW-DFT-D calculations. The results provide a reliable approach for estimation of the stability of MoS2-based heterolayered compounds.

M

organic molecules.18 However, the role of Coulomb attraction between organic and inorganic layers in the formation of NCs based on charged MoS2 layers was never explicitly investigated. Here, we consider the energy required to separate the layers and aim to understand and quantify the contributions of electrostatic factors and specific interactions, such as H-bonding and van der Waals forces, to it. It was shown previously that in the case of the cations produced by protonation of organic bases, the variations in the cation-to-MoS2 ratio and acidity of the reaction medium can give radically different compositions and interlayer distances in the resulting NCs (see ref 19 for phenanthroline- and ref 20 for naphthalene (Naph)-based examples). Such systems can be useful as model compounds, allowing one to distinguish cationspecific and cation-independent stabilization factors. With this target in mind, we synthesized new MoS2−organic compounds with Naph derivatives, a family of compounds exhibiting such phase diversity, 1-naphthylamine (further AN), (naphthalen-1yl)methanamine (AMN), and 1,8-bis(dimethylamino)naphthalene (BDMAN), and studied them using powder Xray diffraction (PXRD), transmission electron microscopy (TEM), and periodic PW-DFT calculations. The NCs were obtained by the previously described method via chemical exfoliation of the precursor compound LiMoS2 in

odifying thin-layer forms of molybdenum disulfide with organic charge-transfer and charge-balancing compounds was shown to produce two-dimensional (2D) materials possessing excellent properties for electronics and optoelectronics,1−3 photo- and electrocatalysis,4 sensors,5 and chargestorage devices.6−8 Such modification can be easily achieved by constructing organic−inorganic nanocrystals (NCs) BnMoS2 resulting from the assembly of negatively charged MoS2 layers and organic cations (B).9,10 These heterolayered NCs, several layers (∼10 nm total) thick and ∼100 nm wide, stabilize the metastable 1T-MoS2 structural modification, necessary for many of the above-mentioned applications, in particular, for the design of highly conductive thin films,11 effective electrocatalysts for hydrogen production from water,12 and supercapacitor electrode materials.8 However, the exploration of properties and applications for MoS2−organic NCs is hindered by the scarcity of data on their structure and the unpredictable nature of the self-assembly process (see refs 13 and 14 for a historical overview of the synthetic efforts). Previous studies aiming to evaluate the effects guiding the formation of heterogeneous MoS2-based materials with specific structure and composition considered the effect of Li adsorption on the structure of 2D MoS2 layer fragments,15,16 studied the nature of charge-transfer interactions between MoS2 and donor organic molecules,17 or attempted to identify individual organic cation−MoS2 interactions in periodic models.9,10 It has been shown that organic surfactants can stabilize water dispersion of monolayer MoS2 particles via electrostatic repulsion between MoS2 flakes covered by charged © XXXX American Chemical Society

Received: November 4, 2016 Accepted: November 30, 2016 Published: November 30, 2016 5162

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The Journal of Physical Chemistry Letters Scheme 1. Synthesis of the Studied NCs

Table 1. Synthesis Conditions, Compositions, and Organic Layer Thicknesses (Δc) for BxMoS2 NCs B AN

pKa of conjugated acid 3.92

AMN

∼9

BDMAN

12.34

guest-to-MoS2 ratio in reaction, mol/mol

pH

Δc, Åa

x, mol/mol

phase

1 3 0.1 1 1 0.1 2

5 2 2 9 2 2 2−8

15.30 7.34 3.76 15.55 7.17 3.66 7.98

0.4−0.6b 0.20 0.10 0.4−0.6b 0.20 0.10 0.20

γ β α γ β α β

a

Calculated as the difference between interlayer distances in the NCs and initial MoS2 (6.15 Å). bThe phase tended to decompose during acidic washing, making it difficult to remove the excess guest from the reaction.

unusual behavior to the different degrees of protonation in the organic layer. Indeed, when the “proton sponge” BDMAN (pKa = 12.3), capable of retaining the proton in a wide interval of pH values, was introduced as the organic guest in the self-assembly process, the β phase was the only product at various pHs. To verify this hypothesis further, we attempted the synthesis of intercalation compounds with AMN, where the amino group is attached to the Naph core via a CH2 linker. This compound has significantly higher basicity than AN (see Table 1). Surprisingly, this cation also formed the same three phases as AN; therefore, the ability to deprotonate at pH ≈ 2 was not relevant for the formation of phases α and β. This inconsistency required a detailed investigation of the stabilization origins in MoS2− organic NCs. Within this study, we could not calculate the thermodynamic stability of the organic−inorganic NCs directly because the environment of the organic cations in solution is generally unknown. The classic lattice enthalpy was also hard to define because the system consists of covalently bound negatively charged layers and molecular cations, which could not be converted to neutral forms without deprotonation of the cation and rearrangement of the MoS2 layer. Therefore, we aimed to use the cohesion energy (EC, estimated as the minimal energy required to separate MoS2 layers assembled in NCs by 40 Å) to evaluate the relative stability of the calculated phases

an aqueous medium to produce monolayer, negatively charged particles of MoS2, which were assembled with organic cations generated in situ by protonation of AN, AMN, or BDMAN molecules (Scheme 1). All other experimental details are presented in the Supporting Information. Depending on the reaction conditions, the assembly of NCs with AN resulted in one of three types of phases (further α, β, and γ) with distinct interlayer distances and organic cation content (Table 1, Figure 1a,b). Initially, we attributed this

EC = min(E40, i − Ecryst) i

(1)

where Ecryst is the energy of a 3D periodic model of a given phase and E40,i is the energy of a given (ith) “separated” model with a forced interlayer distance of 40 Å or larger. Notice that the peeling of some MoS2 layers (essentially the process modeled by these calculations) is observed in the experimental TEM images, and a resistance of a phase to this process is necessary for it to be observable by XRD. We required structural information beyond the layer thicknesses to build models of the bulk state suitable for periodic calculations. Previously, we developed a method9,10 to extract this kind of data from powder patterns of MoS2−organic NCs. These compounds are turbostratically disordered and lack

Figure 1. (a) 00l reflections in the X-ray powder patterns of (AN)xMoS2; (b) schematic representation of the three phases formed by studied compounds (yellow is S, blue is Mo, gray is organic, and a red cross indicates the location of positive charge). 5163

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The Journal of Physical Chemistry Letters

Figure 2. Approximate positions of organic molecules in γ-(AN)0.4MoS2 (a) and β-(BDMAN)0.2MoS2 (b), as determined by their Rietveld fits (below). Semitransparent overlay for γ-(AN)0.4MoS2 indicates possible alternative cation positions.

Figure 3. HRTEM images of (BDMAN)0.2MoS2 particles. FFT images of the framed regions are shown in (a) with an indication of the measured periodicity values. The solid arrows indicate the MoS2 layer fragments detached from the particles. The inset in (b) shows a Fourier-filtered fragment in which the dashed arrows indicate the periodic striped patterns reflecting ordered guest arrangement.

cations in the organic layer via a Rietveld-like fit of their laboratory X-ray powder diffraction patterns. For this purpose, we implemented in Bruker TOPAS21 a modification of the

a “true” 3D periodic structure, with few distinguishable peaks in the powder pattern. Still, we can refine the MoS2 layer geometry, as well as the preferred positions and orientation of 5164

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The Journal of Physical Chemistry Letters “supercell approach” initially proposed by K. Ufer22 for quantitative phase analysis of clays. Here, we applied this modeling to determine the structure of the β and γ phases of the studied compounds because the interlayer distance of the α phases guaranteed that their aromatic molecules laid parallel to the MoS2 sheets. The details of the refinement procedure and the unit cell parameters for studied compounds are presented in the Supporting Information (Figures S1−S5 and Table S1). Judging from the interlayer distance (linear dimensions of the involved molecules are ∼9.0 × 9.0 × 3.4 Å), the γ phases of the unordered AN- and AMN-based hybrid compounds with organic layer thickness exceeding 15 Å should consist of two layers of organic molecules between each pair of MoS2 sheets. Because this phase forms only at pHs higher than pKa of the corresponding conjugated acid, we can assume that the organic layer is only partially protonated and, consequently, only partially charged. According to the Rietveld fit, the organic molecules are inclined relative to the MoS2 plane and are heavily disordered. The deprotonated molecules whose nitrogen atoms are rotated away from the MoS layer may form NaphNH3···NH2Naph hydrogen bonds (Figure 2a). The organic layer structure for the relatively ordered β(BDMAN)0.2MoS2 (Figure 2b) was recovered in detail from the powder diffraction data (see the Supporting Information). The organic molecules are positioned at a sharp angle relative to the MoS2 layers, and the methyl groups of the Me2N···H··· NMe2 fragment fit well into the nanorunnels on the MoS2 surface, resulting in an interaction of this surface with alkyl substituents similar to one in the NCs of MoS2 with tetraalkylammonium compounds.9 Ordered packing of MoS2 layers and BDMAN molecules in the NCs is consistent with the rather regular alternation of the high-contrast MoS2 strips and weak-contrast strips reflecting guest disposition in high-resolution TEM images of (BDMAN)0.2MoS2 (Figure 3a). Notice that the periodicity values measured by fast Fourier transform (FFT) processing of the different image areas are close to the interlayer distance of 14.13 Å as determined by XRD (Table 1). Moreover, close inspection of the Fourier-filtered images indicates a tendency of BDMAN to form rather regular in-layer packing both in between the layers of MoS2 and even on the surface of the NC, clearly visible due to ordered arrangement of corresponding weak-contrast strips (Figure 3b). The positions of organic molecules obtained for the β phase of (BDMAN)0.2MoS2 served as an initial point for quantumchemical modeling of the studied materials. The modeling of the powder patterns for the β phases of (AMN)0.2MoS2 and (AN)0.2MoS2 indicated a similar orientation of the organic molecules to one observed in (BDMAN)0.2MoS2. Therefore, the 3D unit cell from β-(BDMAN)0.2MoS2, adjusted for a correct interlayer distance, was used in subsequent calculations of AN-containing phases. The cations in the calculated α phases were placed between MoS2 layers and rotated to avoid intersections. The hypothetical α-(BDMAN)0.1MoS2 was constructed with an interlayer distance of 10.99 Å, corresponding to an organic layer thickness of 4.84 Å, similar to (N(CH3)4)0.3MoS2.9 Models for separated layers were obtained by setting the unit cell c parameter to ≥40 Å while keeping the organic cations in contact with one MoS2 sheet. Arrangements of cations on the same side of the “free” MoS2 layer (denoted sep) and alternating on both sides (alt), with cations oriented similarly to the periodic model and parallel to the MoS2 layer

( f lat), were tested as starting positions and optimized (Figure 4).

Figure 4. Relative energy (kcal/mol per 10 MoS2 units) and geometry of the studied bulk (left) and separated (right) (AN)0.2MoS2 models.

According to our calculations, the energy of a detached MoS2−organic layer relative to the bulk is dependent on the packing of the organic cations in the hybrid material and on the cations’ arrangement around the MoS2 sheet in the separated state. The calculation results are summarized in Figures 4 and 5 and Tables 2 and S2.

Figure 5. Relative energy (kcal/mol reduced to 10 MoS2 units) and geometry of selected bulk (left) and separated (right) (AN)0.1MoS2 models.

According to these data, the force keeping the NCs together depends on the layer charge and thickness but not on the specific nature of the cation−MoS2 interactions. For example, the cohesion energy is equal for α-AN and α-BDMAN systems (Table 2) despite the differences in the nature of cation-layer contacts (3 N−H···S bonds for AN as compared to several C− H···S contacts for BDMAN). Therefore, the results obtained here for AN apply to its homologue AMN as well. The difference in energy between alt and sep models allows estimation of the cation repulsion in organic layers, and it 5165

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The Journal of Physical Chemistry Letters Table 2. Cohesion Energies EC for Model α and β Phases of Different Composition (kcal/mol reduced to 10 MoS2 units)a (AN)0.1MoS2 (AN)0.1(AN-db)0.1MoS2 (AN)0.2MoS2 (BDMAN)0.1MoS2 (BDMAN)0.2MoS2

α

β

27.1

7.4 16.3 23.7

affected by specific weak cation···cation interactions (such as H···H, π···π, or H···π). A similar lack of specificity can be expected for cation···anion contacts; as we can see, AN and AMN both form similar phases despite significant differences in the acidity of the amino group. The minor impact of specific interactions on the stability of the system also explains the disordered nature of all MoS2−organic composites known to date; Coulomb forces are not affected by lateral layer shifts and rotations. It should be noted that our previous findings concerning the packing of aliphatic cations, particularly Et4N+ and Et2NH2+ ones between MoS2 layers, are consistent with present conclusions. We have shown that these cations occupy the nanorunnels on the MoS2 surface,9 and a cation’s descent into a nanorunnel provides stabilization via a decrease of the interlayer spacing. For aromatic cations not fitting in the nanorunnels, such as AN or AMN, we do not see any effects of the MoS2 surface relief on the cation packing. Thus, our results explain observed dependencies of the MoS2organic NC structure on the reaction conditions as mainly driven by attractive Coulomb interactions between layers and the repulsion of cations within the organic layer. By considering the charge and thickness of the close-packed organic layer and explicitly calculating the PW-DFT cohesion energy in more complex cases, one can now predict the stability of hypothetical MoS2 layered compounds and design new ones.

28.7 51.9

a

Energies for the experimentally observed phases are denoted in bold. b Nonprotonated molecule.

can be as high as ∼20 kcal/mol per organic cation in the β phase of AN (Figure 4). For BDMAN, the energy difference between alt and sep phases is smaller (10 kcal/mol per cation), and the alt−f lat model is only slightly more stable than alt (Table S2). We suppose that the reason here is the delocalization of positive charge within the BDMAN cation, which probably also leads to specific stabilization of organic cation packing in β-(BDMAN)0.2MoS2. We can see that the cohesion energy is a useful proxy for the observed stability of the MoS2−organic heterolayer system; EC is approximately equal for α-AN, β-AN, and β-BDMAN (24− 28 kcal/mol) but significantly higher for β-BDMAN (52 kcal/ mol). This agrees well with experimental results; both α and β phases with an AN cation can be obtained, while BDMAN only forms the β phase, which is almost ordered according to TEM and PXRD data. The closeness of the cohesion energy for αAN0.1MoS2 and β-AN0.2MoS2 despite the difference in composition suggests that the actual self-assembly result will be determined by the amount of available cations. In agreement with this hypothesis, we have found that the formation of a pure α phase with AN or AMN is achieved only with stoichiometric quantities of corresponding organic cations in the reaction medium, while formation of a pure β phase with these cations requires significant cation excess (Table 1). In addition, if we consider a β-AN phase with half of the organic cations deprotonated or absent, the cohesion energies decrease sharply to 16.3 and 7.4 kcal/mol, respectively (Table 2). The comparison of these values provides us with a perspective on the role of uncharged molecules in the organic phase; they are more beneficial for the stability of the structure than empty space, but stabilization provided by cations is considerably higher. In other words, an increase of the interlayer distance without increasing the layer charge decreases the cohesion energy, in agreement with the assumption that the dominant contribution to the stabilization of a heterolayered system comes from the Coulomb forces. As for the γ phase, the repulsion between two layers of cations is likely too strong to form a fully protonated organic layer, but after partial deprotonation, the large interlayer distance is compensated for by higher layer charge. However, during acidic treatment, the neutral molecules are protonated and leave the organic layer, which in turn is destabilized, leading to the experimentally observed irreversible degradation of the corresponding samples. Overall, if all cations in the solution are protonated (and the interaction between them is purely repulsive), the phase allowing maximum charge on MoS2 layers with minimal interlayer distance tends to form. The relatively small cations strongly repel each other within the organic layer, and the composition and structure of such a layer are not significantly

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EXPERIMENTAL METHODS Powder diffraction patterns were measured using a Bruker D8 Advance Vario diffractometer equipped with a Ge(111) Cu Kα1 monochromator and a LynxEye 1D silicon strip detector and refined using Bruker TOPAS 4.2.21 All studied models were optimized at the PW-PBE-D323−25 level in VASP 5.4.1.26−29 Further details are provided in the Supporting Information.

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ASSOCIATED CONTENT

* Supporting Information S

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jpclett.6b02582. Materials synthesis, measurement and structure refinement details, and energies of calculated models (PDF) Experimental structures (CIF) Calculated models (CIF)

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected] (I.S.B.). *E-mail: [email protected] (A.S.G.). ORCID

Ivan S. Bushmarinov: 0000-0002-6534-4133 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the Russian Foundation for Basic Research (RFBR) Grants 14-03-00287-a and 16-29-06184-ofim.

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