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Hydrogen Bond-Driven Self-Assembly between Single-Layer MoS2 and Alkyldiamine Molecules Ivan E. Ushakov, Alexander S. Goloveshkin, Natalia D. Lenenko, Mariam G. Ezernitskaya, Alexander A. Korlyukov, Vladimir I. Zaikovskii, and Alexandre S. Golub Cryst. Growth Des., Just Accepted Manuscript • DOI: 10.1021/acs.cgd.8b00551 • Publication Date (Web): 09 Aug 2018 Downloaded from http://pubs.acs.org on August 10, 2018

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Crystal Growth & Design

Hydrogen Bond-Driven Self-Assembly between Single-Layer MoS2 and Alkyldiamine Molecules

Ivan E. Ushakov, a Alexander S. Goloveshkin,a Natalia D. Lenenko,a Mariam G. Ezernitskaya,a Alexander A. Korlyukov,a Vladimir I. Zaikovskii,b,c Alexandre S. Golub a*

a

A.N. Nesmeyanov Institute of Organoelement Compounds, Russian Academy of Sciences,

Vavilova St. 28, 119991 Moscow, Russia b

Boreskov Institute of Catalysis, Siberian Branch of Russian Academy of Sciences, Lavrentieva

Ave. 5, 630090 Novosibirsk, Russia c

Novosibirsk State University, Pirogova St. 2, 630090 Novosibirsk, Russia

ABSTRACT We report the synthesis, structure determination, and quantum-chemical analysis of a new family of layered nanocrystals (NCs) obtained by a liquid-phase assembly reaction of exfoliated, negatively charged MoS2 sheets with alkyldiammonium ions. A combined PXRD, TEM, FTIR and DFT study allowed us to determine the atomic structure of these turbostratically disordered NCs and to reveal the topology of cation-MoS2 binding interactions. The diamine molecules sandwiched between the sulfur layers of the adjacent 1T-MoS2 sheets were found to interlink these sheets through the hydrogen bonding interaction network. Quantification of these interactions on the basis of the analysis of calculated electron density distribution showed that the strong NH…S bonds contribute 40-80% of the total cation-MoS2 hydrogen bonding interaction energy (33-38 kcal/mol), being accompanied by the contribution of the weaker, but more numerous CH…S bonds. The short-range ordering in the positions of neighboring MoS2 layers was identified and its relationship with organic-inorganic hydrogen bonding was established. DFT based comparison of energetic characteristics for the assembled NCs and their

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delaminated and deprotonated models was performed in order to evaluate stability of NCs against delamination and deprotonation. The data obtained in this study show the prospect for crystal engineering of hydrogen-bonding-based new MoS2-organic nanomaterials.

INTRODUCTION Molybdenum disulfide is a remarkable 2D material attracting tremendous and increasing attention because of its fascinating physicochemical properties addressing the challenges of modern nanoelectronics, photovoltaics and catalysis.

1–4

Great promises for these applications

holds a metastable, not naturally occurring 1T polymorph of molybdenum disulfide, resulting from a negative charge transfer onto the layers of parent, stable 2H polymorph present in the mineral molybdenite.5 A charge transfer induces the change of Mo coordination environment with S from trigonal prismatic (2H) to octahedral (1T) 6–8 and drastically modifies the electronic structure changing its character from semiconducting to metallic.

9–12

Metastability of pure,

neutral 1T modification was evidenced in many studies, its ageing or heating induces restoration of the stable prismatic (2H, 1H) polymorph structure.5,13,14 Structural instability of 1T modification is a serious disadvantage for practical usage, because it may cause time variation of its characteristics. However, keeping a partial negative charge on 1T-MoS2 sheets due to incorporation of organic cations between them was shown to stabilize 1T structure15,16 and, thus, provides a way to involve it in design of various devices. Besides, incorporation of cationic guest species results in the increased interlayer distance between MoS2 layers thus enhancing MoS2 performance for catalyzing hydrogen evolution reaction17 or functioning as effective material for energy storage and supercapacitors.18–20 To date, various MoS2-based hybrid layered nanocrystals (NCs) have been obtained via exfoliation-restacking method, implying exfoliation of Li+(MoS2)- ionic crystals in aqueous media6 and self-organization of molybdenum disulfide monolayers and cationic organic species.15,16,21,22 To favor further rational constructing such heterolayered solids, the knowledge


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of the structure-forming interactions between their components is becoming extremely important. A certain packing of the NCs components is evidently resulted from the combination of different interactions between anionic (MoS2)x- monolayers and organic cations. By analogy with other inorganic-organic systems,23 these interactions can be classified as the nondirectional electrostatic interaction

24

and the directional noncovalent binding, which implies the

participation of the S atoms of sulfide layers. For instance, a set of cation-MoS2 weak CH…S interactions influences the packing of tetraalkylammonium and alkylimidazolium cations, being coupled with π-S (C…S, N…S) interactions in the latter case.16,25 However, a role of strong hydrogen bonds, such as charge-assisted N-H+…(S-Mo-S)- ones, in self-organization of MoS2organic layered crystals was not yet elucidated. Notice that hydrogen bonding of organic molecules with inorganic framework provides the opportunities for tuning the structural and electronic properties of hybrid solids as showed the studies of inorganic–organic perovskites.23,26 In the present study we aimed to clarify an influence of strong and weak hydrogen-bonding on the formation of MoS2-based NCs by synthesizing a new series of NCs with protonated ethylendiamine (EDA), tetramethylethylenediamine (TMEDA) and hexamethylenediamine (HMDA) molecules and analyzing the structures of obtained NCs on the basis of powder X-ray diffraction (PXRD), transmission electron microscopy (TEM), Fourier transform infrared (FTIR) spectroscopy and density functional theory (DFT) calculations data.

EXPERIMENTAL SECTION Synthesis of NCs. The NCs have been synthesized as described elsewhere16 using the singlelayer aqueous dispersions of MoS2.6 For this purpose, purified natural molybdenum disulfide (99.7 %, DM-1, Scopin Factory, Russia) with a particle size (95%) smaller than 7 µm was treated with an excess of 1.6 M n-butyllithium solution in hexane (Aldrich) for 1 week, then washed with hexane and dried in vacuum. The obtained crystalline compound, LiMoS2, was immersed 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 hydrochlorides of diamines were prepared by dissolving ethylenediamine dihydrochloride (98%), or hexamethylenediamine (98%), or tetramethylethylenediamine (99%, all purchased from Aldrich) in water acidified by HCl. In each case, 20 mL of the solution containing the diamine in an amount of 3 mol/mol of MoS2 was added to 200 mL of the MoS2 dispersion and the reaction mixture was stirred on a magnetic stirrer for 2 h. Then, the precipitate formed during the stirring was collected by centrifugation, washed by water and dried in vacuum. The pH of the first supernatant was measured using a Hanna Instruments 8424 pH meter. The composition of the products was determined from elemental analysis data (C, H, N, Mo). Characterization 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. ATR FTIR spectra were measured on a Vertex 70v (Bruker) Fourier spectrometer with a resolution of 4 cm-1 using a GladiATR unit with a diamond working element. All necessary corrections were made using a OMNIC program package. Elemental analysis of the products was performed on a Carlo Erba Elemental Analyzer 1106 (C, H, N) and on a VRA 30 Carl Zeiss X-ray fluorescent spectrometer (Mo). TGA was performed on a Derivatograph-C instrument (Hungarian Optical Works) in the temperature range 20-700oC at a heating rate of 10 °C/min under argon flow of ca. 100 ml/min. The 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, in transmission between Kapton films. The measurement range was 6-120° 2θ, and the


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step size was 0.00917° 2θ. All modeling and indexing was performed using TOPAS 4.2 software. The details of the structure refinement procedure are described in the Supporting Information. Quantum-Chemical Calculations. DFT (PW-PBE-D) periodic calculations were performed in VASP package.27–30 Projector augmented wave (PAW) pseudopotentials31,32 were used for all atoms to describe core electrons. The contribution of valence electrons was described as series of plane waves with kinetic energy cutoff 545 eV. Exchange and correlation terms of total energy were described by PBE functional,33 with Grimme D3 van der Waals correction for dispersion interactions.34 Electron density function for topological analysis was obtained in separate single point calculations with higher kinetic energy cutoff (1360 eV) and dense grid for Fast Fourier Transformation (the distance between grid points is shorter than 0.035 Å). Topological analysis was carried out by using AIM program, a part of ABINIT software.35 Other details of DFT calculations and quantum theory of atoms in molecules (QTAIM) analysis are presented in the Supporting Information.

RESULTS AND DISCUSSION

Main Structural Features of NCs. Interaction of single-layer dispersions of MoS2 sheets produced by exfoliation of crystalline LiMoS2 in water 6 with diamine molecules (B) at the pH conditions ensuring domination of di-protonated form of corresponding diamine in solution resulted in self-assembly of nanocrystalline solids according to the reaction pathway expressed in equation 1. The found product compositions given in Table 1 are consistent with incorporation of organic molecules in the combined structures in essential amount, which falls in the range typical for incorporation of organic molecules between MoS2 sheets (0.1-0.3 mol/mol MoS2).16,42,43 B/H+

H2 O

Li+(MoS2)- → [Li+ + (MoS2)x- + (1-x) OH-]aq → (BH22+)xMoS2 ACS Paragon Plus Environment

(1)


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-(1-x)/2 H2

Table 1. Used diamine molecules, their pKa values, the pH of reaction media and the experimental (x) and theoretically modeled (xmod) diamine-to-MoS2 molar ratios in the obtained NCs

a

Diamine NH2CH2CH2NH2 (EDA)

pKa1, pKa2 9.92; 6.86

pH 3-6.5

x, a 0.17

x mod, 0.167

(CH3)2NCH2CH2N(CH3)2 (TMEDA)

10.14; 8.26

5

0.11

0.125

NH2CH2(CH2)4CH2NH2 (HMDA)

11.86; 10.76

9

0.11

0.125

Corresponds to 6.2 (EDA) and 7.5 (TMEDA or HMDA) % mass.

TEM study showed that the assembly process yields the flat, varying in thickness (from several to tens nanometers) and lateral dimension (up to ~1 µm) particles with alternating stacking of MoS2 sheets and organic layers (Figures 1, S6, S7), similarly to the MoS2 hybrid layered compounds previously obtained by restacking method.16,18,21,36,37 In the assembled structures, the interlayer periodicity in the c direction (perpendicularly to the layers) exceeds that in the starting MoS2 (~6.2 Å) by the thickness of individual organic layer as shown by the images of the side projections in Figures 1c, S6c and S7c where the dark and bright strips correspond to the layers of MoS2 and organic substance, respectively. Typically, the MoS2 sheets are spaced from each other by the organic layers approximately equal in thickness as follows from both the analysis of the intensity profile normally to the layer plan and the FFT processing of the image area (Figure 1d), though a small number of lesser and greater interlayer distances can also be detected in the NCs.


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Figure 1. TEM images of NCs TMEDA-MoS2, showing the survey views of the particles (a,b), typical side projection (c), and intensity profile along the line on “c” (d). The inset in “c” shows FFT of the framed region.

The observed regularity in the layers disposition along the c direction in TEM images is consistent with the presence of sharp 00l reflections in powder X-Ray diffraction patterns of NCs (Figures 2, S1, S2), which allows this periodicity averaged over the most ordered areas of the NCs to be measured. By analyzing the broadening of these reflections by the Scherrer method,38 the average crystallite sizes of NCs perpendicularly to the layers were evaluated to be 14.2 nm, 18.3 and 39.5 nm for NCs with HMDA, TMEDA and EDA, respectively (see Supporting Information for the details).



Figure 2. Powder X-ray diffraction pattern of (TMEDA)0.11MoS2 and its fit. Only the main reflections are designated.

Characterization of NCs by FTIR spectroscopy confirmed the diamine molecules immobilization in them. For instance, in the spectrum of TMEDA-MoS2 shown in Figure 3 the main absorption bands of pure TMEDA observed in the area of N-H and C-H deformations can be identified, including the intense bands at 1456, 1263 and 1030 cm-1and a number of weaker bands. Besides, the FTIR spectra showed that the amino-groups of diamines are protonated as follows from the appearance of a wide absorption beginning from 3700 cm-1 and extending up to 2500 cm- 1, typical of protonated N-H groups; this strong absorption masks the bands of C-H stretches of pure TMEDA in the 3000-2800 cm-1 range. Similar spectral features were observed for NCs with EDA and HMDA (Figures S3, S4).

Figure 3. FTIR spectra of MoS2, TMEDA and NCs TMEDA-MoS2. The obtained NCs were stable at room temperature for a long time, however, they underwent gradual decomposition on heating as evidenced by their TGA and DTG curves (Figure S5). The weight loss occurs most rapidly at ~150oC for NCs with EDA and at ~250 oC for NCs with HMDA and TMEDA, and approaches the value expected for full removal of organic component at 500-600oC, which is typical for MoS2-based compounds with various organic guest molecules.39,40


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Phase analysis performed on XRD patterns of reaction products showed no reflections belonging to the parent MoS2 (JSPDF #77-1716) thus indicating its full conversion to new phases in the course of exfoliation-restacking procedure. The layered character of these new phases evidenced by TEM study and primary XRD data as well as the protonated state of incorporated diamine species revealed by FTIR measurements confirm that the assembly process proceeds according to equation 1 and results in hetero-layered NCs.

Atomic Structure Determination of NCs. As for other previously studied restacked MoS2based compounds,15,16,25,41 the measured XRD patterns of NCs show well distinguishable signatures of turbostratic disorder in the layer stacking, evinced by a presence of broad, asymmetric hk0 reflections and an absence of hkl ones (Figures 2, S1, S2). Though the atomic structure determination for such substances cannot be achieved by routine powder diffraction analysis, their structures can be solved using a supercell approach previously developed by K. Ufer for turbostratic clays.42 Recently, we have successfully adapted this method for MoS2based compounds with organic guests.25 The essence of the approach consists in describing the diffraction pattern of turbostratic compound with a unit cell consisting of one layer of the refined structure represented by single MoS2 sheet with associated organic molecules, and numerous empty layers. This makes possible full profile description and Rietveld-like refinement of the atomic structure within a structural model. Short-range correlations in the positions of adjacent MoS2 sheets in the structure can be accounted for by using two layers of the refined structure in the supercell, for which the shift of these layers relative to each other is refined.16,43 Finely, the structural models simplified by the absence of disorder were optimized by carrying out quantumchemical DFT (PBE-D3/PW) calculations. In these models, the protonated diamine molecules were placed at the positions obtained in structure refinement and regularly distributed on the sulfide sheets. The content of diamines in these models indicated in Table 1 (xmod) thus can thus be considered as corresponding to the theoretical composition of NCs. Notice that the



experimentally found and theoretically modeled compositions are close to each other for all NCs. Other details of powder patterns modeling and structure refinement are presented in the Supporting Information (Figures S10, S11 and Tables S1- S4).

MoS2 Sheet Geometry. X-ray structure determination revealed that in all three studied compounds the MoS2 monolayer geometry corresponds to 1T-MoS2 modification, which is characterized by the octahedral surrounding of Mo with S, non-equivalent Mo…Mo distances, formation of zigzag chains by Mo atoms along the b axis and crimping of the sheet surface at a nanoscale (Figure 4). The determined parameters of MoS2 sheet structure in the NCs are given in Table 2. This geometry is unlike the geometry of starting MoS2 used for synthesis of NCs, representing 2H modification of this material with the trigonal prismatic Mo-S coordination, equal Mo…Mo distances and atomically flat sheets. The lattice parameters of MoS2 superstructure in the NCs (a × b) relates to the lattice parameter of the hexagonal unit cell of 2HMoS2 (a0) as a ≈ √3a0, b ≈ a0.

Figure 4. The structure of TMEDA-MoS2 viewed in the b (left) and a (right) directions. The shortest binding CH…S and NH…S contacts are shown with indication of distances (Å) and interaction energies for NH…S bonds (kcal/mol).


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Table 2. Selected Structural Parameters of NCs

Parametera a (Å) b (Å) c (Å)

EDA a = 5.7002(10) b = 3.2253(6) c = 9.7073(1)

TMEDA a = 5.693(4) b = 3.229(2) c = 9.833(7)

HMDA a = 5.650(3) b = 3.228(2) c = 9.968(4)

Mo-Mo (Å)

2.8584(5); 3.2253(6); 3.7093(5)

2.876(12); 3.229(2);3.686(12)

2.800(15); 3.228(2); 3,729(17)

Mo-Mo-Mo (deg)

68.7(2)

68.3(3)

70.4(4)

S-S ∆ɀ1, ∆ɀ2 (Å)

3.43(3), 2.64(2)

3.42(4), 2.63(4)

3.40(3), 2.438(19)

0.40(2)

0.40(4)

0.48(2)

0.775(6) 0.787(4)

0.855(6) 0.674(6)

0.762(6) 0.684(5)

Height variation in S positions on the sheet surface (Å) Interlayer shift (∆) along a axis (Å) along b axis (Å) a

α = β = γ = 90°. The presence of 1T-MoS2 in the NCs structure also agrees well with the zigzag-like

periodic distortions of nanocrystal lattice clearly seen on the Fourier-filtered TEM frontal projections of NCs, accompanying by the appearance of superstructure reflections in corresponding FFT images (Figure S8). Besides, on the TEM side projections of the thinnest, few-layer NCs particles, the specific saw-like contrast theoretically simulated for 1T-MoS2 layer stacking in earlier work 44 can be recognized (Figure S9). As was mentioned above, pure 1T-MoS2 is metastable and tends to transform to stable 2H form.5,13,14 Stabilization of 1T-MoS2 in the present compounds can only be explained by fixation of residual negative charge on their MoS2 sheets due to combination with cationic species counterbalancing this charge. Indeed, the formal excess negative charges present on MoS2 layers are 0.34 (EDA) and 0.22 (HMDA, TMEDA) е- per MoS2 formula unit according to the determined content of di-protonated diamines derivatives. Notice that in the case of EDA the charge value is close to the greatest one (~0.35 е-/Mo) reported for MoS2-organic nanocrystals with long-chain alkylammonium cations.45



Disposition of Diamine Molecules. The NCs structures suggest that the nanorelief of MoS2 monolayers strongly affects a disposition of diamine species in the interlayer space. As can be seen in Figure 5, in all cases they occur inside the “nanovalleys” formed by the sulfur atoms. Obviously, the embedding of protonated diamines molecules in the valleys allows strongest approachment of the cations and anionic MoS2 sheets, forced by the Coulomb interaction. In the case of EDA and TMEDA, this attraction results in the nearly smallest possible interlayer distances (c) expected for NCs with given organic molecules from structural consideration. For HMDA, however, the c-value is not so small as would be expected by analogy with its lower homolog, EDA; the HMDA layer is thicker by ~0.3 Å (Table 2). This means that HMDA molecules avoid the orientation expected from the only Coulombic interaction, suggesting that hydrogen bonding also strongly affects the NCs structure.

Figure 5. Comparison of EDA (a), TMEDA (b) and HMDA (c) molecules disposition on MoS2 sheet. The “nanoridges” (peach) and “nanovalleys” (blue) on the sheet surface are colored. Hydrogen atoms are omitted.

To compare the organic molecules disposition, we considered the angles formed by the vectors, connecting two main coordinating centers in molecules, N1-N2, with the plane of MoS2 layer (χ) and with the valley’s direction, i.e., with the b-axis (φ) as well as the dihedral angle


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between the planes of C-N trans-zigzag and MoS2 layer (ψ) (see Figure 6 for schematics). Corresponding values summarized in Table 3 show that all diamine molecules are similarly stretched out roughly in parallel to the valleys with the φ angles being as small as 6o (EDA, TMEDA) and 16o (HMDA). All the molecules demonstrate slight inclination to the MoS2 layer plane, expressed by the values of χ ranging from 13 to 23o. However, HMDA differs from two other diamines by greater tilt of its carbon-nitrogen molecular plane relative to MoS2 layers (ψ ≈ 75o instead of ~25o in case of EDA or TMEDA), which explains the greater interlayer distance observed for HMDA compound.

Figure 6. Schematic representation of the angles χ, φ, ψ characterizing diamines disposition in NCs with indication of the plane of MoS2 (grey), the plane of C-N trans-zigzag (hatched), the molecular axis N1-N2 and its projection onto the plane of MoS2 (N1’-N2’).

Table 3. Values of Characteristic Angles of Organic Cation Disposition in NCs Angle, deg. χ φ ψ

EDA 23.5 6.3 25.1

TMEDA 14.7 6.4 25.8

HMDA 13.4 16.1 74.8

Binding Interactions between MoS2 and Protonated Diamine Molecules. To reveal the intermolecular binding interactions between the components of assembled NCs, the structures optimized by DFT calculations were analyzed in the framework of Bader’s quantum theory ACS Paragon Plus Environment


(QTAIM) as described in the Supporting Information. The sets of the (3,-1) bond critical points (bcp) found by the topological analysis of the electron density distribution ρ(r) convincingly show that the protonated diamines form two types of binding contacts with adjacent MoS2 sheets, the NH…S and CH…S ones. This is illustrated in Figures 4, S10, S11. It should be noted that, though the both revealed types of binding interactions are widely classified as nonconventional H-bonds, they differ considerably with each other and with classical OH…O hydrogen bonds in the proton acidity and acceptor center basicity and in the relative role of dispersion and electrostatic interactions in their stabilization.46,47 For CH…S contacts in CH4…H2S molecular complexes, even domination of van-der-Waals type of binding was claimed.48 To classify the revealed interatomic interactions by their strength, a correlation between the density of the local potential energy at the critical point and the energy of corresponding contact, previously established for hydrogen bonds in work,49 was then applied. The resulting interaction energies (Econt) of individual contacts grouped by their type (NH…S and CH…S) and the total energies of cation – MoS2 interaction are presented in Table 4 and Tables S2,S3,S4. The obtained total energy values fall in the relatively narrow interval 33-38 kcal/mol. Of the two H-bond types, the NH…S ones are expectedly much stronger. As evidenced by an example of EDA, the individual binding energy of such contact can be as high as ~13 kcal/mol and their mean energy (4.3 kcal/mol) is comparable with the energy of corresponding interactions between organic amines and such S-acceptor as, for instance, Me2S.50 It is important to note that each of two ammonium groups of organic molecule is involved in strong NH…S bonding with one of the opposite sulfide layers, which explains an inclination of molecules with respect to MoS2 layers revealed in structural study. The mean energies of CH…S interactions are much smaller, 0.8-1.3 kcal/mol, but their cumulative contribution to the total interaction may be significant and amount to ~20% (EDA), ~40% (HMDA) or even to ~60% (TMEDA), depending on how many such contacts are allowed


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by the molecule’s geometry. For instance, the structure of TMEDA with four CH3-substituents at N atoms and two methylene units is more favorable for the formation of such bonds than the structure of HMDA with the same total number of carbon atoms arranged in the methylene chain.

Table 4. Energies (kcal/mol per Mole of Cation) and Numbers of NH…S and CH…S Bonding Interactions in NCs

Contact NH…S CH…S Cation…MoS2 NH…S CH…S Cation…MoS2

NH…S CH…S (all) CH…S (CH3-groups) Cation…MoS2

Number of contacts EDA 6 5 HMDA 4 18

TMEDA 2 21 15

Econt 25.9 6.7 32.6 22.9 15.2 38.1

16.0 21.9 12.6 37.9

Correlations in MoS2 Sheet Stacking. Powder patterns modelling evidenced that the stacking of MoS2 layers in the present series of NCs is not completely random similarly to some previously solved MoS2-based structures.16 A pairing of the layers in the structural models was therefore applied to reveal the relative shift of the adjacent layers. Dissimilarity of the refined shift values for different NCs (Table 2) begs the important question of what influences the tendency of the adjacent MoS2 sheets to certain local ordering in each case. It appears that the cause refers to the matching of diamine species geometry to MoS2 sheet surface, needed for the formation of the strongest specific interactions, i.e, the NH…S bonds. Indeed, the distance between two nitrogen atoms in diamine molecule (referred to as d, see Table 5) and the S-S distance in the valley’s direction (equal to the parameter b of the lattice, ≈ 3.23 Å) are not multiple of each other for all diamines, with the misfit of +15 for EDA, +16% for ACS Paragon Plus Environment


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TMEDA and -11 % for long HMDA molecule (as compared with the 3b dimension in the latter case). The misfit (M) is evaluated here as M = min [(d – nb)/d]·100, where n is an integer number providing nb dimension closest to d. Notice that the importance of lattice-matching between MoS2 and molecular species was early reported for molecular epitaxy on MoS2 surface.51 The incommensurability with sulfur sublattice is thus unfavorable for strong hydrogen bonding of the both amino-groups of diamine molecule with only one of the adjacent MoS2 layer. However, the molecules can effectively form strong hydrogen bonds with the S atoms of different layers if they are shifted properly with respect to each other upon assembling process. For instance, if we account for the layer shift along the b axis (∆b) in a bilayer structure, the distance between the S atoms available for coordination in this bilayer become equal to nb +/- ∆b (along the b axis in the projection onto the ab plane). To estimate the misfit for this type of coordination we should modify the above expression for M by introducing the layer shift (∆b). The resulting equation 2 is suitable for evaluation of the misfit values for both types of coordination: with one sulfide layer (∆b = 0) or with two sulfide layers (∆b ≠ 0). M = min {[d – (nb +/- ∆b)]/d}·100

(2)

As can be seen in Table 5, the layer shift along the b axis diminishes the misfit to small values, approaching the coordinating species to commensurability. These observations explain why different ends of diamine molecules are strongly bound with opposite sulfide layers and, more generally, evidence that the short-range correlations in layer stacking of NCs are caused by the strong organic-inorganic hydrogen bonding.

Table 5. Matching of Molecules Dimension N1-N2 (d) to Sulfur Lattice Periodicity along the b Axis for Bilayer Structure of NCs Taking Account of Layer Shift (∆b) Distance d, Å nb +/- ∆b, Å Misfit, %

EDA 3.81 4.01 (b+∆b) -5.2

TMEDA 3.83 3.90 (b+∆b) -1.8


HMDA 8.75 8.97 (3b-∆b) +2.5

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Evaluation of NCs Stability against Delamination and Deprotonation. A comparison of the energetic characteristics computed by DFT method for the assembled and delaminated layered system can be used for evaluation of the cohesion or exfoliation energy for intercalated MoS2 24 and layered double hydroxides.52 We used similar approach to calculate the cohesion energy (Ec) for EDA-MoS2 structure by the equation 3, defining Ec as the minimal energy needed to separate the organic-inorganic layers of nanocrystal. Ec = E40 – Ecryst

(3),

where Ecryst and E40 are the total energies of the assembled EDA-MoS2 compound and its delaminated model, respectively, calculated using DFT. As with previous studies, delamination was simulated by an increase of the interlayer distance to 40 Å.24 Delaminated models, differing in disposition of organic molecules (Figure S12), were constructed and optimized by DFT calculations to select the model with minimal energy as described in the Supporting Information. The value of Ec equal to 22.5 kcal/mol (per mole of EDA) was thus obtained, showing the energy advantage of assembly process for this compound. Next, we tested the stability of EDA-MoS2 NCs against discharge of MoS2 layers coupled with deprotonation of diammonium EDAH22+ cations, which may occur without or with participation of molecular oxygen according to the reaction equations (4) and (5) or (6) and (7), respectively. For this purpose, the corresponding reaction energies (Er) were calculated (Table 6) using the energies of the reactants (Table S5). The models of the partly and fully deprotonated NCs needed for this determination have been constructed through modification of the parent structure by removal of the certain number of hydrogen ions and optimized.

(EDAН2)1/6MoS2 → (EDAН)1/6MoS2 + 1/12 H2

(4)

(EDAН)1/6MoS2 → (EDA)1/6MoS2 + 1/12 H2

(5)

(EDAН2)1/6MoS2 + 1/24 O2 → (EDAН)1/6MoS2 + 1/12 H2O

(6)



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(EDAН)1/6MoS2 + 1/24 O2 → (EDA)1/6MoS2 + 1/12 H2O

(7)

As can be seen in Table 6, the both steps of deprotonation of the assembled NCs in the absence of oxygen are characterized by the positive energy change, suggesting disadvantage of these reactions. This is consistent with incorporation of diammonium forms of molecules in the NCs. As to the oxidation-induced deprotonation, this process is only favorable for the reaction (5), transforming di-protonated species to mono-protonated ones. Full discharge of the cations and sulfide layers under action of oxygen is also not expected. The results obtained by these calculations thus predict the stability of present NCs and allow one to add them to the family of MoS2-organic compounds promising for application in electrocatalysis, sensors and electronic devices.

Table 6. Reaction Energies (Er) for Deprotonation of (EDAH2)1/6MoS2 (in kcal/mol Reduced per Mole of Cation) Product (EDAН)1/6MoS2

(EDA)1/6MoS2

Reaction 4 6 5 7

Er 10.7 -18.3 31.1 2.2

CONCLUSIONS We have shown for the first time that layered nanocrystals containing MoS2 monolayer sheets interlayered with protonated diamine molecules are readily formed by the reaction of the components in solution. According to the structure determination results obtained by the powder XRD pattern modelling, DFT calculations, FTIR and TEM study, the organic-inorganic hydrogen bonding interaction is an important structure-forming factor in the present nanoorganized hetero-layered systems. The strong NH…S interactions contribute to their stabilization, being accompanied by the contribution of weaker, but more numerous CH…S interactions. The later impact depends on the nature and geometry of alkyl fragments in molecule ACS Paragon Plus Environment

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and can be comparable in magnitude with that of strong hydrogen bonding. Evaluation of the NCs stability predict their resistance to delamination and deprotonation. The geometry adapted by the MoS2 layers is similar for all three studied NCs. It is modified by the charge transfer to the sheets of starting MoS2, resulting in their change to 1T structure type, promising for many important applications. The results presented in this paper are believed to be of considerable interest for crystal engineering of new MoS2-based organic-inorganic nanomaterials.

ASSOCIATED CONTENT Supporting Information The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.cgd.xxxxxxx. Additional characterization data; structure refinement and calculations details; CIF files of the refined structures.

AUTHOR INFORMATION Corresponding Author *E-mail: [email protected]. ORCID Alexandre S. Golub: 0000-0002-3105-3506 Notes The authors declare no competing financial interest.

ACKNOWLEDGEMENTS The funding of the work by the Russian Foundation for Basic Research (grant 16-29-06184) is gratefully acknowledged. The X-ray and FTIR measurements were performed using the facilities of Center for Molecular Composition Studies of INEOS RAS.



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For Table of Contents Use Only Hydrogen Bond-Driven Self-Assembly between Single-Layer MoS2 and Alkyldiamine Molecules

Ivan E. Ushakov,a Alexander S. Goloveshkin,a Natalia D. Lenenko,a Mariam G. Ezernitskaya,a Alexander A. Korlyukov,a Vladimir I. Zaikovskii,b,c Alexandre S. Golub a*

a

A.N. Nesmeyanov Institute of Organoelement Compounds, Russian Academy of Sciences,

Vavilova St. 28, 119991 Moscow, Russia b

Boreskov Institute of Catalysis, Siberian Branch of Russian Academy of Sciences, Lavrentieva

Ave. 5, 630090 Novosibirsk, Russia

TOC Graphic

Synopsis Protonated diamine molecules sandwiched between the adjacent 1T-MoS2 sheets interlink these sheets through hydrogen bonding interaction network and influence the short-range ordering of sulfide layers in resulting nonocryctals (NCs). Quantification of these interactions and stability of the NCs against structure perturbations are discussed.


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Hydrogen Bond-Driven Self-Assembly between ... - ACS Publications

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