Structural Properties and Phase Transition of Exfoliated-Restacked

Article pubs.acs.org/JPCC

Structural Properties and Phase Transition of Exfoliated-Restacked Molybdenum Disulfide Alexander S. Goloveshkin, Ivan S. Bushmarinov,* Natalia D. Lenenko, Mikhail I. Buzin, Alexandre S. Golub, and Mikhail Yu. Antipin A.N. Nesmeyanov Institute of Organoelement Compounds, Russian Academy of Sciences, 119991, Vavilova St., 28, Moscow, Russia S Supporting Information *

ABSTRACT: The product of exfoliation and restacking of MoS2 in acidic conditions is studied in detail using X-ray powder diffraction, transmission electron microscopy (TEM), thermogravimetric analysis (TGA), and differential scanning calorimetry (DSC). The temperature dependence of powder patterns reveals that the heating of exfoliated-restacked MoS2 is a way to a new nanostructured MoS2-based layered material that remains nanosized even upon heating to 850 °C. Previously this material has been described as 2H-MoS2, but according to the X-ray diffraction (XRD) data, its structure cannot be correctly described by any of the “usual” MoS2 polytypes. A model of the structure of the material describing its XRD patterns and thermal behavior is discussed in detail.

1. INTRODUCTION The molybdenum disulfide MoS2 is a naturally occurring layered solid that finds applications in industry in both its bulk and dispersed forms. The material consists of covalently bonded MoS2 layers, which are linked together by van der Waals interactions.1 The ability of these layers to move relatively easily against each other makes MoS2 an important tribomaterial,2 and MoS2-based composites are also used in petrochemical catalysis.3 Recent studies have shown that materials based on nanodispersed MoS2 demonstrate better characteristics in both tribochemical and catalytic applications.3,4 Recently, the nanoparticles of MoS2 have been produced using sulfidation of different precursors, including pure Mo,4 its oxides,5 ammonium molybdate,6 Mo(CO)6,7,8 MoCl4 in micellar solutions,9 and the gas phase,10 and also by laser ablation of crystalline MoS2.11 One of the important and mild methods for producing MoS2 nanoparticles is single-layer dispersion of MoS2 crystals, which implies the reduction of starting material to (Li)+(MoS2)− followed by detachment of its layers from each other in aqueous solvents.12 The acidic treatment of the single-layer dispersed system leads back to MoS2, but the properties of the precipitate differ significantly from those of initial bulk MoS2 due to irregularities of the stacking process and excess charge left on the MoS2 layers: the product most likely corresponds to the formula HxMoS2.13 The UV-viz studies14 and the ability of the product to form thin films demonstrate that the material obtained in this way consists of medium-sized nanoparticles.15 The products of MoS2 restacking have been reported to form typical hexagonal 2H-MoS2 upon heating,13 but the details of the phase transition and the further thermally driven structural evolution of this material have never been published. © 2013 American Chemical Society

In the current manuscript, we report the study of the structure of freshly prepared and annealed material resulting from restacking of exfoliated MoS2 in acidic conditions (“HxMoS2”; due to unresolved questions of its composition, we will further refer to it as “E-R MoS2”) and discuss the phase transition occurring in this substance.

2. EXPERIMENTAL SECTION 2.1. Materials Preparation. A powder of purified natural MoS2 (DM-1, Scopin factory, Russia) with a particle size (95%) smaller than 7 μm was used as a starting material. The samples of nanodispersed MoS2 were obtained by exfoliation and restacking of this material using the procedure outlined in eq 1. nBuLi

MoS2 ⎯⎯⎯⎯⎯→ Li+(MoS2)− H 2O

⎯⎯⎯→ [Li+ + (MoS2 )x − + (1 − x)OH−]aq H+

⎯→ ⎯ MoS2

For this purpose, initial MoS2 was intercalated with lithium by treating it with an excess of a 1.6 M n-butyllithium solution in hexane (Aldrich) for one week followed by washing with hexane and vacuum drying. The obtained product, Li1MoS2, was exfoliated in bidistilled water or acetonitrile−water (10% vol.) solvent under ultrasonication to form MoS2 single-layer dispersions as described in ref 16. Concentrations of the dispersions were 1, 3, and 10 g L−1. Both the aqueous and aqueous−organic dispersions were acidified by addition of HCl in amounts required for the pH of aqueous dispersion to be Received: January 3, 2013 Revised: March 26, 2013 Published: March 27, 2013 8509

dx.doi.org/10.1021/jp400087c | J. Phys. Chem. C 2013, 117, 8509−8515

The Journal of Physical Chemistry C

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adjusted to 2 or 3, and the reaction mixtures were stirred during 20 min on a magnetic stirrer. After that, the formed precipitates were isolated by centrifugation, washed three times with water (or acetonitrile−water mixture), and dried in vacuum. The pH values were controlled in the filtrates of aqueous dispersions using a Hanna Instruments 8424 pH meter. To test the action of oxidizers, FeCl3 and KMnO4 were added to the aqueous dispersions together with HCl solution required for the pH 2 to be reached in amounts of 10 and 3 mol/mol of LiMoS2, respectively. 2.2. Thermal Treatment. An amount of 300 mg of the obtained material was loaded in a quartz ampule with length of 30 cm and diameter of 21 mm, and the ampule was connected to a vacuum pump, evacuated, and inserted in a furnace previously heated to the desired temperature. The materials were heated in a stepwise manner at the temperatures 100, 250, 400, 550, 700, and 850 °C, during 2 h at each temperature. 2.3. Methods. Elemental analysis (S, Mo) of the obtained materials was performed on a VRA 30 Carl Zeiss X-ray fluorescent spectrometer. All the samples were found to contain sulfur and molybdenum in the atomic ratio S/Mo = 2.00 ± 0.03 that corresponds to the stoichiometry of MoS2. The DSC measurements were done in a Mettler DSC-822e differential scanning calorimeter at a heating rate of 10 °C/min in argon atmosphere. TGA was performed on the “Derivatograth-C” (MOM, Hungary) in argon at a heating rate of 5 °C/min on a sample of about 10 mg by weight. The powder X-ray diffraction (PXRD) patterns of MoS2 nanoparticles were obtained in transmission mode, with the studied compound deposited between two kapton thin films. The measurements were performed on a Bruker D8 Advance diffractometer (Vario geometry) equipped with a Kα1 Ge(111) focusing monochromator and a LynxEye 1D position-sensitive detector. The measurement range was 7−90° 2θ for heated and aged samples and 7−65° 2θ for freshly precipitated samples, and the step size was 0.0105° 2θ. The wide peak of Kapton at 22.770(2)° 2θ does not overlap with peaks of MoS 2 nanoparticles. High-resolution transmission electron microscopy (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. Particles to be examined by TEM were deposited on a perforated carbon film mounted on a copper grid.

Table 1. Particle Size (D002) and Interlayer Distances in the Samples of MoS2 Nanoparticles synthesis conditions sample 1 2 3 4 5

solvent CH3CN/H2O (10/1) H2O H2O H2O H2O

pH

C (g/L)

D002 (nm)

Layers’ number

d (Å)

−

3

5.0

8

6.235(2)

2 3 2 2

3 3 1 10

11.5 12.3 10.2 11.4

18 20 16 18

6.2401(3) 6.2525(3) 6.2712(3) 6.2552(2)

Figure 1. Diffraction patterns of the MoS2 nanoparticles. Precipitated sample 1 (black); sample 1 after heat treatment at 850 °C (red); and crystalline MoS2 (blue).

formalism.18 The instrument broadening was measured using the NIST SRM 640c silicon powder. The obtained crystallite sizes, in effect, were calculated according to the Scherrer formula (eq 1).

D=

Kλ β cos(θ)

(1)

where β is the integrated breadth of the peak in radians; K is a constant (0.89); θ is the diffraction angle; and λ is the Cu Kα1 wavelength (1.540596 Å). We have used the 002-line of MoS2 to measure the particle size in direction perpendicular to the layers of MoS2 (D002). The particle sizes and interlayer distances obtained by the 002 peak fitting are represented in Table 1. As can be seen from these results, the interlayer distance in all studied samples is by ∼0.1 Å longer than that in crystalline MoS2 (6.15 Å). Since d-spacing for the 002 line is the interlayer distance of MoS2 (further d), the value of D002 reflects the number of layers in the E-R MoS2 nanoparticles. The number of the layers in the particles, estimated as D002/d, is ∼8 for the sample obtained in acetonitrile (sample 1) and 16−20 for the samples precipitated in water (samples 2−5). Variation of concentration and acidity of the medium at the stage of precipitation only slightly affects the characteristics of the samples (Table 1). Thus, the nature of the solvent has the greatest effect on the dispersion of the material. The fact that MoS2 particles precipitated in acetonitrile are noticeably smaller than the particles obtained in water can be explained by a difference in ability of these solvents to prevent aggregation of ultradispersed particles formed primarily in the processes of dispersion of initial crystalline MoS2. Better stabilization of the smallest-size MoS2 nanoparticles in acetonitrile as compared to water is consistent with recent UV−viz spectral data obtained for suspensions of restacked MoS2 in corresponding solvents.14

3. RESULTS AND DISCUSSION 3.1. Dependence of MoS2 Particle Size on Synthesis Conditions. The samples of E-R MoS2 prepared in different conditions (Table 1) were studied by PXRD. All lines in their diffraction patterns were strongly broadened, and the peak positions loosely corresponded to those of crystalline 2H MoS2 (Figure 1). The overall broadening of the lines indicates that the particle size in the samples is relatively small: due to strong peak overlap, the particle size could be estimated only for the 002 reflection (2θ = 14.6°). The d-spacings for all reflections in the freshly precipitated E-R MoS2 were systematically larger than those in crystalline MoS2. For the further analysis, the peak fitting of the powder pattern was performed in the TOPAS software package,17 the peaks were defined in terms of fundamental parameters, with Lorenzian crystallite size refined according to the Balzar 8510



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3.2. Structural Rearrangements in the Freshly Prepared MoS2 Nanoparticles. We have studied the changes in the obtained nanosized MoS2 during the heat treatment. The powder patterns of the freshly precipitated E-R MoS2 contained a number of peaks at 2θ ≥ 30°, whose positions do not correspond to those of initial 2H-MoS2. These reflections are strongly broadened and overlap. Thus, the structure of the freshly prepared samples is strongly disordered and possibly exhibits peaks belonging to a superlattice, which is consistent with previous studies.13 After heating at 100 °C for 2 h, the general outline of the pattern changes significantly (Figure 2).

2H-MoS2 (6.15 Å) and to decrease gradually during annealing. The temperature dependence of d, however, has a maximum at T = ∼100 °C observed for both the samples prepared in acetonitrile and in water (Figure 3). It could be assumed that this maximum appears due to a phase transition. To check this suggestion, the samples of E-R MoS2 were studied by thermal analysis (TGA, DSC). Indeed, the DSC graph demonstrates an exothermic transition in the wide temperature range with peak maximum at 105 °C (ΔH = 150 J/g). (Figure 8, curve 1), supporting the phase transition hypothesis. It should be noted that TGA does not reveal any noticeable weight changes at this temperature. The repeated DSC scan does not show any heat release, indicating that the phase transition is irreversible (Figure 8, curve 2). Similar phase transitions were observed at 98 °C for MoS2 restacked in Li-containing dispersions in alkaline environment13 and at 95.7 °C for 1T-MoS2 obtained by oxidation of K0.33(H2O)0.6MoS2.19 The nature of this phase transition, however, is not entirely clear. The fact that it is observed in the powder pattern indicates that it involves a structural change in the E-R MoS2. Since the powder pattern after heating resembles one of the 2H MoS2 and contains fewer peaks, we can expect that the freshly precipitated E-R MoS2 contains structural distortions which are removed during the phase transition. Since the distortions manifest themselves in the hk0 area of the powder pattern and only slightly affect the 002 peak, they are definitely internal to the MoS2 layers and do not affect their stacking. Different models for intralayer changes in E-R MoS2 had been discussed in the literature, including but not limited to octahedral distortions of MoS6 polyhedra,19 formation of regions with octahedral MoS6 polyhedra,20 shortening of Mo−Mo distances,21,22 and formation of “zigzag” superstructures.23 Most models of this kind correspond to structural changes within the layers of MoS2, and we expect that the three-dimensional structure of bulk E-R MoS2 is even more complicated due to stacking defects. Indeed, the TEM data show that the stacking of particles (and most likely MoS2 layers) in E-R MoS2 is almost random (Figure 4), and the interlayer distance can vary within one nanoparticle of MoS2 (Figure 5).

Figure 2. Diffraction patterns of freshly precipitated E-R MoS2 (sample 1) before (black line) and after (green line) heating to 100 °C.

All peak positions of the new pattern can be described as the lines of crystalline 2H-MoS2 (space group P63/mmc). However, the parameters of the hexagonal cell, particularly the c one, are slightly larger than in crystalline MoS2. The lines are broadened, while less than that in the freshly precipitated E-R MoS2. The interlayer distance in the particles of the precipitated samples determined as a half of the unit cell dimension along the c-axis was found to be higher than that in

Figure 3. Temperature dependence of the interlayer distance. Blue line, sample 1; red line, sample 2. 8511



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Figure 4. Seemingly random stacking of particles in bulk E-R MoS2 (sample 1), top view. Figure 6. Patterns of E-R MoS2 sample 1 heated to 100 °C (green line), E-R MoS2 precipitated in the presence of Fe3+ (red line), and KMnO4 (black line) and aged E-R MoS2 (blue line).

Figure 5. E-R MoS2 particle (sample 1), 0001 viewing direction. The rim of the MoS2 flat particle is bent over, showing the number of layers.

While we cannot obtain the exact model for the intralayer MoS2 distortions from the PXRD data, we can detect their presence from the additional peaks in the patterns of freshly precipitated MoS2. The PDF data on restacked WS224 as well as electron diffraction studies for MoS2 and WS2 show that the metal coordination polyhedra within the layers of restacked layered disulfide species are significantly distorted. One of the sources of these distortions is excess negative charge on MoS2 layers, as has been shown by PDF studies on LiMoS2.25 Since all additional peaks disappear upon heating, we can conclude that the phase transition is at least partially associated with removal of this excess charge remaining from the exfoliation process. Indeed, the powder patterns of “aged” E-R MoS2 (exposed to air for 30 days; see Experimental Section) and one precipitated in the presence of oxidizers are very similar to those of E-R heated to 100 °C (Figure 6). Thus, the formula of freshly precipitated E-R MoS2 can be described as HxMoS2 with H+ cations balancing some negative charge, in agreement with ref 13. This agrees with the recently reported findings that the freshly precipitated E-R MoS2 can be intercalated by cations but loses this ability upon heating to 150 °C.26,27 The DSC data also confirm this hypothesis, as the heat released upon heating of the aged sample (102 J/g) is much smaller than for the fresh one (150 J/g) (Figure 8, curve 3). Still, the ordering process in E-R MoS2 is not associated exclusively with charge removal. It is more visible from the DSC curve for an aged sample that some ordering process continues after 100 °C too, and this process is also irreversible (Figure 8, curves 3 and 4). In the X-ray diffraction patterns, it manifests itself both in shortening of the interlayer distance

Figure 7. Heat treatment of the E-R MoS2 (sample 1, precipitated from CH3CN/H2O).

(Figure 3) and in decrease of the peak widths (Figure 7), suggesting the aggregation of nanoparticles upon heating. These processes are in our opinion independent from the phase transition at 100 °C and occur continuously upon heating from 25 to 850 °C, as can be seen from the sharpening of the 002 peak in PXRD patterns (see Figure 7). 3.3. Crystallite Size Dependence on the Heat Treatment. The pattern of the material obtained upon heating of E-R MoS2 has significant anisotropic peak broadening, indicating that the material may be more complex than the

Figure 8. DSC curves for E-R MoS2 (sample 1) fresh (1, 2) and aged for 45 days (3) at first (1, 3) and second (2) heating at a heating rate of 10 °C/min in argon. 8512



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modeling with different fault probability performed in ref 28 allowed the authors to conclude that their system corresponded to 40% fault probability (p). The modeling result unfortunately could not be directly compared to our data due to the differences in the interlayer distance (see Figure 3). The powder pattern of E-R MoS2, however, bears strong similarity to the result of the modeling with p = 50% (Figure 10),

2H-MoS2. When expressed in terms of the Scherrer formula, the anisotropic broadening allows us to formally “measure” the “crystallite size” in different directions, corresponding to the sizes of coherent scattering regions, which can be smaller than the particle size in the case of internal disorder. As was discussed above, we used the 002-line of MoS2 to measure the particle size in a direction perpendicular to the layers of MoS2 (D002). Most structural defects within the MoS2, as well as stacking errors, do not affect the interlayer distance, thus the D002 can be interpreted as the thickness of the nanoparticle (Figure 9).

Figure 10. Modeling of powder pattern for 2H-MoS2 with 50% 3Rlike stacking probability according to ref 28 (red line) and experimental powder pattern for E-R MoS2 heated to 100 °C (black line).

Figure 9. Crystallite sizes in the different crystallographic directions and the corresponding planes in a fragment of 2H-MoS2 structure with 3R-like stacking faults. A,A*, 2H-MoS2 cell; ABC, 3R-MoS2.

The crystallite size obtained from 100 line (D100) shows the size of the ordered (long-range correlated) regions parallel to the MoS2 layers. As can be seen from Figure 9, it still will not be affected by the most common stacking errors. To estimate the particle size in a direction inclined to the layers (Figure 9), the line 103 was chosen (D103). This line should be affected by all kinds of intralayer and stacking defects. Indeed, the D103 remained the smallest one at all temperatures, while the particle size increased in all directions upon heating from rt to 800 °C (Figure 11, see also Table S1 in the Supporting Information). If the nanoparticles could be described as pure 2H-MoS2, they could not be smaller in the dimension inclined to the MoS2 layers than in the directions both perpendicular and parallel to the layers. Thus, the material obtained by heating of E-R MoS2 has significant intrinsic disorder, a crude model of which, in terms of 3R-like stacking faults, is demonstrated in Figure 9. It should be noted that MoS2 has three polytypes, 1T, 2H, and 3R, with the two latter differing only by the layer stacking. Namely, 2H exhibits antiparallel stacking with two layers in a unit cell rotated by 60° against each other, and 3R is characterized by parallel stacking of layers with a shift by 1/3 of the a lattice vector (see Figure 9). Thus, we define the 3Rlike stacking fault as a parallel stacking of a layer within a 2H structure. Such stacking faults can be anticipated in a material formed by precipitation of single MoS2 layers from a solution. The rhombohedral 3R-polytype of MoS2 is normally observed only under high pressure, but 3R-like stacking faults have been recently found to be common in MoS2 and WS2 nanotubes and fullerene-like structures.28 The powder pattern

Figure 11. Temperature dependence of E-R MoS2 particle size in various crystallographic directions. A corresponds to sample 1 precipitated from CH3CN and B to sample 2 precipitated from H2O.

indicating that during the restacking the orientation of the stacked layers is almost random. Importantly, the line broadening observed for E-R MoS2, particularly the relative broadening of the 103 line, is also reflected in the modeling result. In this work, we cannot conclude that our system can be fully described by a mix of 2H- and 3R-like stacking of ideal MoS2 layers; we are, however, convinced that the 3R-like stacking faults are very common in the heated E-R MoS2. Since the stacking is the main factor determining the polytype of MoS2, the studied material should be denoted neither as a 2Hnor as a 3R-one. The particle size increase upon heating becomes more pronounced closer to 800 °C, as the system approaches the temperature of solid-state synthesis of MoS2 from Mo and S (700−1000 °C)29,30 and the particle ordering speeds up. Notably, even particles heated to 800 °C remain nanostructured, not exceeding 40 nm in any direction. The smaller, CH3CN-precipitated particles of sample 1 agglomerate faster, converging to similar values of D to sample 2 at 700 °C. In our 8513



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Notes

opinion, the increase of D002 is mostly caused by agglomeration of different layered fragments, and the increase in D100, which effectively starts only at 550 °C, is caused by additional intralayer ordering. This additional ordering, in our opinion, cannot be caused by excess charge on MoS2 layers, which should be absent at these temperatures. Compared with ordering in the [002] and [100] directions, the increase of the coherent scattering region size in the [103] direction would require a more complicated process consisting of simultaneous moving and rotation of the MoS2 layers as a whole. For that reason this process is expected to have higher activation energy and to require more elevated temperature to happen at reasonable speed. It should be noted that upon heating starting from 550 °C the powder patterns of E-R gradually lose similarity with the results of modeling as 2H-MoS2 with 3R-like stacking faults reported in ref 28, as the line at 33−35° 2θ begins to split, contrary to the model powder pattern. The powder pattern at 850 °C approaches one for 2H-MoS2. The anisotropic broadening, however, remains pronounced (see Figure 7 and Figure 9). This fact indicates that the disorder in the studied material is possibly more complex than modeled in ref 28. One source of the line broadening not completely removed by heating can be layer buckling: the relatively long interlayer distance in heated MoS2 (Figure 3) also supports the presence of such defects.

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This manuscript is dedicated to Professor Mikhail Yu. Antipin, who passed away on 18th February 2013. The authors thank Dr. V. I. Zaikovskii of the Institute of Catalysis, Novosibirsk, for performing the TEM measurements and Dr. Lothar Houben of Peter Gruenberg Institut, PGI-5, Research Centre Juelich GmbH, Juelich, for providing data of Debye formula modeling (see Figure 10). This work was supported in part by the Russian Foundation for Basic Researches (11-03-00922, 11-030652-a and 12-03-90016-Bel).

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CONCLUSIONS The exfoliated MoS2 restacked in acidic conditions (“HxMoS2”) is demonstrated to be a highly disordered metastable material, the ordering of which can be irreversibly restored by oxidation or heating to 100 °C. However, not all disorder in the system can be attributed to the excess charge on MoS2 layers, and contrary to previously published data the ordering process does not stop after heating to 100 °, and continues to temperatures as high as 800 °C. The resulting material, even after heating to 800 °C, cannot be described as 2H-MoS2 reported previously: the uneven broadening of diffraction peaks in the powder pattern of this material shows a significant amount of internal disorder, most likely stacking faults. The comparison with independent modeling results28 indicates the possibility that this new material has equal probabilities of 2H- and 3R-like stacking and thus should not be referred to as either one. It should also be noted that the interlayer distance in the material after phase transition at 100° exceeds that for crystalline 2HMoS2 by 0.11 Å and does not reach the ideal value of 6.15 Å even at 800 °C, confirming the hypothesis that this material is significantly different from 2H-MoS2. This previously unreported MoS2 material remains nanostructured with crystallite size less than 40 nm at temperatures as high as 800 °C, which makes it an interesting candidate for the applications where a thermally stable nanosized MoS2 is required.

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ASSOCIATED CONTENT

S Supporting Information *

The table of MoS2 particle sizes in different crystallographic directions (used in Figure 11). This material is available free of charge via the Internet at http://pubs.acs.org.

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REFERENCES

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Structural Properties and Phase Transition of Exfoliated-Restacked

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