Two-Dimensional Nanosheets and Layered Hybrids of MoS2 and

Dec 23, 2013 - Ammoniated MS2 (M = Mo,W) have been synthesized by reacting LixMS2 with a saturated solution of ammonium chloride. While, largely ...
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Two-Dimensional Nanosheets and Layered Hybrids of MoS2 and WS2 through Exfoliation of Ammoniated MS2 (M = Mo,W) A. Anto Jeffery, C. Nethravathi, and Michael Rajamathi* Materials Research Group, Department of Chemistry, St. Joseph’s College, 36 Lalbagh Road, Bangalore 560 027, India S Supporting Information *

ABSTRACT: Ammoniated MS2 (M = Mo,W) have been synthesized by reacting LixMS2 with a saturated solution of ammonium chloride. While, largely neutral NH3 is present in the interlayer of ammoniated MoS2, equal amounts of NH3 and NH4+ ions are present in the tungsten analog. The ammoniated MS2 exfoliate readily in a variety of polar solvents with exfoliation being best in water. Ammoniated WS2 forms a more stable colloidal dispersion compared to the Mo analog because the ammonium ions do not deintercalate easily from the layers even on exfoliation. The dispersions are comprised of large nanosheets of MS2 with lateral dimensions in the order of micrometers. The layers could be restacked from the colloidal dispersions by evaporating the solvent. While the colloidal dispersions of ammoniated MoS2 yield NH3-free MoS2 on restacking, the dispersions of ammoniated WS2 yield WS2 intercalated with NH4+ ions. Co-stacking of MoS2 and WS2 nanosheets from a mixture of both the colloidal dispersions results in MoS2−WS2 hybrids in which the MoS2 and WS2 nanosheets are randomly stacked. Photoluminescence measurements of MS2 nanosheets and MoS2−WS2 hybrids indicate phase stability and existence of direct bandgap. using scotch tape method,2 poor scalability and lack of control over thickness limits this technique. Alternatively, one can scale-up the quantity of 2D nanosheets to higher yields by exfoliating ion-intercalated TMDCs in solvents (chemical exfoliation).13−15,31,32 Besides the high yield of 2D nanosheets, solvent exfoliation has a few additional advantages in the ease of preparation of nanocomposites, layered hybrids, and fabrication of thin films.6,20−24 Conventionally, TMDCs are exfoliated by intercalation of lithium metal followed by reaction with water.12−14 The intercalated lithium metal reacts violently with water producing hydrogen gas causing the MX2 layers to separate, thus resulting in colloidal suspension of MX2 layers in water. The exfoliated nanosheets obtained by this method differ from those obtained by mechanical exfoliation in terms of structure and electronic properties.33−38 Major drawbacks associated with this method are that LixMS2 compounds are extremely air sensitive, flammable, and react aggressively during exfoliation. Alternative intercalants that can be encapsulated in TMDCs, which can induce exfoliation in a variety of solvents but are less sensitive to ambient conditions, are the need of the hour. Ammoniated MS2 could be ideal precursors for liquid phase exfoliation since the intercalated NH3/NH4+ would invite solvent molecules into the interlayer and the evolution of ammonia gas during sonication would assist the exfoliation

1. INTRODUCTION Exfoliation of layered solids1 is of immense interest as it yields free-standing (two-dimensional) 2D nanosheets of mono atomic/molecular layer that have been exploited as potential materials for diverse applications.2−7 Layered solids such as graphite and graphite oxide,8,9 smectite clays,10 boron nitride,11 metal oxides,5 layered double hydroxides,4,12 and transition metal dichalcogenides (TMDCs)6,13−19 have been widely investigated for their exfoliation property. Exfoliation of TMDCs such as MS2 (M = Mo, W) and MSe2 (M = Mo, W, Nb, Ta) to acquire monolayer or multilayer nanosheets is important as these 2D materials find application in the fields of electronics, optoelectronics and energy storage applications.20−24 TMDCs are made of a stacking of graphene-like layers of the formula MX2 in which M can be any metal of IV, V, and VI group transition elements forming hexagonal layers sandwiched between two sheets of X, where X is a chalcogen (X = S, Se, Te). These TMDCs can be metallic or semimetallic depending on the type of metal coordinated to the chalcogen.25,26 Monolayer or few layer stacked TMDCs can be derived using different techniques such as mechanical (scotch tape method)2 or chemical exfoliation,6,13−19 thermal ablation method,27 chemical vapor deposition (CVD) on substrates,28,29 and direct solvothermal synthesis.30 Among the available methods, exfoliation is the most suitable route for the large scale production of nanosheets.6,13−19 Though high-purity ultrathin flakes have been obtained by exfoliating bulk crystals of MoS2 through mechanical cleavage © 2013 American Chemical Society

Received: November 6, 2013 Revised: December 21, 2013 Published: December 23, 2013 1386

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Ammonium chloride (99.8% pure) and all organic solvents employed in this work were procured from Merck India Ltd. All these chemicals were used as supplied without any further purification. 2.1. Synthesis of Ammoniated MoS2. Dry MoS2 (1 g, 6.24 mmol) was stirred with 8 mL of a 2.5 M solution of BuLi in hexane under nitrogen atmosphere at room temperature for 48 h to form lithiated MoS2 (LixMoS2). After 48 h, the reaction vessel was cooled in ice to ∼5 °C and 75 mL of freshly prepared aqueous saturated solution of ammonium chloride was injected into the reaction vessel. The slurry obtained was vacuum filtered using G3 sintered crucible fitted with Whatman filter paper to remove n-hexane and other soluble products and washed six times with rectified spirit and dried at room temperature to get a black powder (ammoniated MoS2). 2.2. Synthesis of Ammoniated WS2. Dry WS2 (1 g, 4.03 mmol) was taken in a 10 mL round-bottom flask fitted with a septum. Ten milliliters of 2.5 M solution of BuLi was injected into the flask. The flask containing WS2 and BuLi was then transferred into a Teflon lined autoclave (80 mL capacity) containing 10 mL of dried n-hexane under nitrogen atmosphere. The rubber septum was removed and the autoclave was sealed immediately. The autoclave was heated at 90 °C for 24 h in an air oven. After 24 h, the autoclave was opened under nitrogen atmosphere. Seventy-five milliliters of freshly prepared aqueous saturated solution of ammonium chloride was directly poured into the product mixture containing lithiated tungsten disulfide (LixWS2). The slurry obtained was processed as in the case of ammoniated MoS2 to get a black powder (ammoniated WS2). 2.3. Stability Studies of Ammoniated MS2. As we noticed that the ammoniated MS2 samples were unstable and over a period of time they converted into MS2, we followed the stabilities of the ammoniated MS2 on standing for a few days. After each day, the powder X-ray diffraction (XRD) patterns of the ammoniated samples were recorded to ascertain the presence of intercalated species. The samples were also heated at 120 °C for 4 h under nitrogen atmosphere and the XRD patterns of the heat-treated samples were recorded. 2.4. Exfoliation of Ammoniated MS2 in Various Solvents. About 75 mg of ammoniated MS2 was treated with 25 mL of water and the mixture was sonicated (35 kHz) at room temperature for 2 h. The resulting colloidal dispersion was centrifuged at 1000 rpm for 10 min to remove any undispersed solid. The undispersed solid was washed several times with acetone, dried, and weighed to find out the amount of dispersed solid. To investigate the stability of colloidal dispersion, the dispersion was allowed to stand undisturbed for several days. At the end of each day, the solid settled was separated by centrifugation, washed, dried, and weighed. This procedure is repeated until the total weight of the settled solid was greater than or equal to 50% of the weight of the dispersed solid, and the time taken for ∼50% of the dispersed solid to settle down is taken as the direct measure of colloidal stability and is represented as half-life (t1/2) of the colloidal dispersion. The dispersion behavior in various organic solvents was studied through similar experiments. In circumstances where the stability of the colloidal dispersion was poor, the settled solids were separated on hourly basis. 2.5. Restacking of MS2 Layers from the Colloidal Dispersions. The dispersed layers of MS2 could be restacked by slowly evaporating the solvent from the dispersions. The layers could also be restacked by adding acetone to the

process. One of the disadvantages of LixMS2 is that the dispersion of the monolayers of MS2 obtained from them would be contaminated with lithium compounds like LiOH and films or composites prepared using these dispersions would need extensive cleaning to get rid of these impurities.38 In the case of ammoniated MS2, the intercalant ammonia would escape from the dispersion during exfoliation and even if it had not escaped, the composites and films obtained from these dispersions can be made ammonia-free by heating at temperatures less than 250 °C. While ammoniated MS 2 (M = Ti,Ta) have been prepared39,40 way back in 1970s, the analogs of Mo and W have so far not been reported.40 The earlier attempts to make these compounds were dependent on the direct reaction between MS2 and liquid ammonia and the failure of this reaction to yield ammoniated MoS2/WS2 was attributed to the inability of the intercalating species to donate electrons to the filled valence band of the 2H polytype of these solids.40,41 It is well-known that Li can be intercalated in these solids and theoretical studies and experimental evidence suggest that the intercalation is accompanied by a change in the crystal symmetry of these solids from 2H to 1T.33−38 It has also been shown that NH4+/RNH3+ could be intercalated in these solids through partial ion exchange of Na+ in NaxMoS2 or when the exfoliated layers of these solids were restacked in the presence of amines.41,42 Thus it should be possible to prepare ammoniated MoS2 and WS2 by reacting LixMS2 with a source of NH3/NH4+ ions. Fabrication of field effect transistors based on van der Waals heterostructures of 2D layers of h-BN, MoS2, or WS2 as tunnel barriers with graphene serving as one/two electrodes43,44 has triggered a lot of interest in terms of design and synthesis of heterostructures of 2D layers. Similarly, alignment of monolayers of similar lattice constants, like MoS2 and WS2, is expected to exhibit optical and electronic properties distinct from its components.45 Currently, the reported heterostructures are assembled by isolating layers from the bulk using scotch tape method. Efforts are in progress to grow layers one on top of the other by CVD methods. However, the CVD method has limitations because the weak interlayer interaction generally favors island growth rather than that of continuous monolayers.46 Alternatively, it should be possible to use colloidal dispersions of 2D nanosheets of MoS2/WS2 to fabricate layer-by-layer deposition using Langmuir−Blodgett technique or by self-organizational assembly (flocculation)4,5 of a mixture of suspensions of different 2D layers. Colloidal processing has been demonstrated to be a scalable approach in the synthesis of superlattice like assemblies in the case of other 2D layered systems.4,5 In this paper, we report a method to synthesize NH3/NH4+ ion intercalated layered MS2 (M = Mo,W) compounds. The ammoniated MS2 are reasonably stable in ambient conditions and exfoliate readily in a variety of solvents to give stable dispersions. Layered hybrids of MoS2−WS2 of varying compositions (MoS2/WS2) were synthesized by flocculating a mixture of the colloidal dispersions of ammoniated MoS2 and WS2 in different compositions. Photoluminescence of the monolayers and the hybrids have also been examined.

2. EXPERIMENTAL SECTION MoS2 and WS2 (both 99% pure) were procured from Sigma Aldrich, Germany and 2.5 M solution of n-butyllithium (BuLi) in n-hexane was supplied by Across Organics, U.S.A.. 1387

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Figure 1. Schematic illustration of the steps involved in the synthesis, exfoliation and restacking of ammoniated MS2 (M = Mo, W).

dispersions. The solids settled within 24 h were washed with aceone and dried at room temperature. 2.6. Costacking of MS2 Layers from the Colloidal Dispersions; Formation of MoS2−WS2 Hybrids. MoS2− WS2 hybrids of varying compositions (25:75, 50:50, 75:25 mol ratios of MoS2/WS2) were prepared by mixing the required volumes of colloidal suspensions of nanosheets of MoS2 and WS2 and the mixture was sonicated for 1 h. The volume of the resulting mixture of colloidal dispersion was reduced to one tenth its initial volume by rotary evaporation of the solvent. The remaining solvent was slowly evaporated at 65 °C. The solid product was washed with acetone and dried at 65 °C to constant mass. 2.7. Characterization. All the samples were analyzed by recording XRD patterns using PANalytical X’pert pro diffractometer (Cu Kα radiation, secondry graphite monochromator, scanning rate of 1° 2θ/min). The XRD patterns of the colloidal dispersions of ammoniated MS2 were recorded at a lower scan rate of 0.4° 2θ/min. Infrared (IR) spectra of samples were collected using Nicolet Impact 400D FTIR spectrometer using KBr pellets at 4 cm−1 resolution. Thermogravimetic (TG) analysis was performed using SII EXSTAR6000 TG/DTA6200 and differential scanning calorimetry (DSC) was performed using EXSTAR X-DSC7000. Transmission electron microscopy (TEM) images were recorded using a JEOL F3000 microscope operated at 300 kV. The nanoplatelets of MS2 obtained through exfoliation were characterized by atomic force microscopy (AFM) using APER A100 atomic force microscope in tapping mode. Sample for AFM analysis was prepared by spin coating an aqueous colloidal dispersion of MS2 onto a Si substrate. Xray photoelectron spectroscopy (XPS) measurements were carried out with an ESCALab220i-XL spectrometer using a twin-anode Al Kα (1486.6 eV) X-ray source. All spectra were calibrated to the binding energy of the C1s peak at 284.51 eV. The base pressure was around 3 × 10−7 Pa. Raman and photoluminescence (PL) spectra were recorded using a Horiba Jobin-Yvon T6400 Raman spectrometer at excitation laser wavelength of 488 and 514 nm.

Figure 2. XRD patterns of pristine MoS2, ammoniated MoS2 and aqueous colloidal dispersion of ammoniated MoS2 (a−c); pristine WS2, ammoniated WS 2 and aqueous colloidal dispersion of ammoniated WS2 (d−f).

the case of ammoniated MoS2 (Figure 2b), the basal spacing increases by ∼3.3 Å compared to the pristine MoS2 (Figure 2a). In the case of ammoniated WS2 (Figure 2d), the increase in basal spacing is slightly higher at ∼3.7 Å compared to its precursor WS2 (Figure 2c). The increased basal spacing matches with the size of NH3/NH4+ ion whose diameter is ∼3.5 Å. The increase in the basal spacing observed here is comparable to that observed in the case of ammoniated layered chalcogenides reported previously.40,41 In addition, both the ammoniated samples show extensive stacking disorder as can be seen from the merging of all high-angle reflections into two sawtooth shaped broad reflections starting at 2θ = ∼32 and 57° indicative of turbostratic disorder.47,48 This is expected since intercalation in MoS2/WS2 is accompanied by a change in the crystal symmetry. The mechanism of formation of ammoniated MS2 is as follows. The unreacted BuLi and LixMS2 react violently with aqueous solution of ammonium chloride causing partial decomposition of ammonium chloride, thus releasing NH3 which is intercalated within the MS2 layers along with NH4+ ions. The reactions can be summarized as follows

3. RESULTS AND DISCUSSION The steps involved in the synthesis and exfoliation of ammoniated MS2 are schematically depicted in Figure 1. The XRD patterns of the ammoniated MS2 (M = Mo,W) samples compared with those of the pristine MS2 in Figure 2 indicate intercalation of ammonia species in both the cases. In

NH4Cl → NH3 + HCl 1388

(1)

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Table 1. Composition Analysis of the Ammoniated MS2 Samples mass percent TG mass loss (%) sample

total ammonia % (by titration)

NH3+H2O (first mass loss)

NH4+ (second mass loss)

nominal formula

ammoniated MoS2 ammoniated WS2

8.51 ± 0.08 12.0 ± 0.1

9.2 5.9

0.8 6.0

(NH3)0.98(H2O)0.07(NH4+)0.08MoS2 (NH3)0.85(NH4+)0.86WS2

shows two endothermic peaks at two different temperatures corresponding to the mass loss steps. It has been shown in the case of ammoniated TiS2 that the two endothermic steps correspond to the deintercalation of NH3 (around 100 °C) and NH4+ ion (around 240 °C).49,50 Thus we can say that in ammoniated MoS2 the intercalated species is largely NH3 and in the case of ammoniated WS2 the interlayer has both NH3 and NH4+ ions. Chemical composition of the ammoniated MS2 was further analyzed by X-ray photoelectron spectroscopy (XPS). In the case of ammoniated MoS2, Mo 3d, and S 2p spectra (Figure 4a,b) correspond to Mo4+ and S2− of 2H polytype of MoS2. The N 1s spectrum (Figure 4c) indicates the presence of NH3. Thus, the intercalation of the neutral NH3 molecules in MoS2 is not accompanied by a change in the polytype.38 The expanded Mo 3d and S 2p spectra (Supporting Information, Figure S1) exhibit low intensity peaks corresponding to Mo6+ and oxidized S indicating minimum oxidation during intercalation. W 4f and S 2p spectra (Figure 4d,e) of ammoniated WS2 suggests that intercalation of NH3 and NH4+ ions in WS2 is accompanied by structural changes. As summarized in the table (Figure 4), 2H and 1T polytypes coexist in the ammoniated sample.38 Because WS2 is more prone to oxidation than MoS2,15 the presence of WO3−xSx (Figure 4d and Supporting Information, Figure S2) is significant. The N 1s (Figure 4f) spectrum indicates the presence of both NH3 and NH4+ ions. The ammoniated MS2 samples slowly lose the intercalated ammonia with time. In Figure 5A, we show the evolution of XRD pattern of ammoniated MoS2 with time when stored in ambient conditions. There is no discernible change in the pattern till the sixth day of storage. On day seven, we can see the reflections due to the free (unintercalated) MoS2 emerging, though the major component is still the ammoniated solid. The amount of free MoS2 increases with time and by the fifteenth day, reflections due to the ammoniated solid are almost absent. The deintercalation could be hastened by heating the sample.41 When heated at 120 °C for 4 h, ammoniated MoS2 loses all the intercalated species to yield NH3-free MoS2. This is in accordance with the observation that in the thermogravimetry curve of ammoniated MoS2 (Figure 3a) the single mass loss is complete by around 120 °C. The time-dependent XRD patterns of ammoniated WS2 under ambient conditions are shown in Figure 5, panel B. In contrast to ammoniated MoS2 the tungsten analog is quite stable at room temperature. The XRD pattern recorded after 15 days is identical to that recorded on day 1. The 00l reflections get broader after a month and the broadening increases with time. However, even after 3 months we do not observe reflections due to pure WS2. The intercalated species could be removed by heating the ammoniated WS2 at 120 °C for 4 h. Even at this temperature the deintercalation is not complete and a disordered WS2 is the product. This observation is in accordance with the fact that the intercalated ammonium ions are lost only at ∼200 °C as shown by thermogravimetric analysis (Figure 3c).

LixMS2 + y NH3 + z NH4 + + x HCl → (NH3)y (NH4 +)z MS2 + x LiCl +

⎛x⎞ ⎜ ⎟H ⎝2⎠ 2

(2)

The intercalated NH3/NH4+ species in the ammoniated MS2 samples were estimated by two methods. In the first method, the sample was heated at 250 °C under nitrogen flow and the evolved vapors passed into HCl solution. By back-titration, the amount of ammonia liberated was calculated. NH3/NH4+ in the samples were also estimated by thermogravimetric (TG) analysis of the samples. The composition data of the samples are presented in Table 1. From this data, it is clear that in ammoniated MoS2 the intercalated species is largely NH3 while in ammoniated WS2 both NH3 and NH4+ are intercalated in equal amounts. In addition to indicating the total amount of intercalated species, TG curves of the ammoniated samples (Figure 3a,c)

Figure 3. TG (a,c) and DSC curves (b,d) of ammoniated MoS2 and ammoniated WS2, respectively.

suggest the presence of two types of intercalated species that deintercalate at two different temperatures. In the case of ammoniated MoS2 (Figure 3a), we observe a single step mass loss centered at 80 °C while in ammoniated WS2 (Figure 3c) there are two distinct mass loss steps centered at 100 and 240 °C. The DSC curve of ammoniated MoS2 (Figure 3b) presents a single endothermic peak while ammoniated WS2 (Figure 3d) 1389

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Figure 4. XPS spectra showing Mo 3d (a), S 2p (b), N 1s (c), and W 4f (d), S 2p (e), N 1s (f) core level peak regions of the as prepared ammoniated MoS2 and ammoniated WS2 respectively. The binding energies are summarized in the table.

nonpolar solvents. The most stable dispersion is formed in water with the t1/2 value being 4 days, highest among all the solvents studied. The amount of ammoniated MoS2 dispersed per liter was found to be highest in formamide though the stability of the dispersion is less than that obtained in water. Exfoliation was also good in polar solvents like dimethylsulfoxide, 1-hexanol, 1-octanol, and 1-octanethiol. Exfoliation in 1octanethiol is of particular interest because this oxygen-free solvent would prevent oxidation of the reactive monolayers of MS2. The polar nature of the interlayer due to the intercalated NH3 allows polar solvents to enter the interlayer leading to exfoliation. Complete exfoliation of ammoniated MS2 was confirmed by the XRD analysis of their colloidal dispersions in water (Figure 2c,f). Complete absence of 00l reflections and the presence of broad in-plane 2D reflections in the XRD patterns of the dispersions suggest that the dispersions comprise monolayers of MS2. The exfoliation behavior of ammoniated WS2 is almost similar to that of the Mo analog with a few deviations. Ammoniated WS2 too exfoliates only in polar solvents and shows negligible exfoliation in nonpolar solvents. In most of the solvents, the amount of ammoniated WS2 dispersed is less than that of the Mo analog but the dispersions are more stable. The lower stability of colloidal dispersions of ammoniated MoS2

Figure 5. XRD patterns of (A) ammoniated MoS2 kept standing for various durations at room temperature (a−e) and the same heat treated at 120 °C for 4 h (f); (B) ammoniated WS2 kept standing for various durations at room temperature (a−d) and the same heat treated at 120 °C for 4 h (e).

Ammoniated MS2 were exfoliated in various solvents under mild sonication. The amount of ammoniated MS2 dispersed through exfoliation in different solvents and the t1/2 values of the dispersion are shown in Table 2. From these values, it is clear that ammoniated MoS2 exfoliates to form colloidal dispersion in polar solvents and it does not exfoliate in 1390

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Table 2. Summary of the Exfoliation Behavior of Ammoniated MS2 in Various Solvents number of millimoles of MS2 delaminated per dm3 of the solvent solvents water formamide dimethylsulfoxide acetonitrile ethanol 1-butanol 1-octanethiol toluene carbon tetrachloride cyclohexane

ammoniated MoS2 6.1 11.5 9.9 1.0 1.5 0.9 2.7

± ± ± ± ± ± ± 0 0 0

ammoniated WS2

0.3 0.6 0.5 0.1 0.1 0.1 0.2

5.0 7.0 4.7 0.7 1.8 1.7 0.8

± ± ± ± ± ± ± 0 0 0

0.4 0.2 0.3 0.1 0.2 0.3 0.2

t1/2 of the colloid (h) ammoniated MoS2 96 12 12 4 12 12 8

± ± ± ± ± ± ±

8 1 1 1 1 1 1

ammoniated WS2 146 144 24 12 4 3 4

± ± ± ± ± ± ±

6 8 1 1 1 1 1

may be due to the fact that NH3 molecules weakly bound to the MoS2 layers get released slowly with time leading to weakening of the solvation of layers. In the case of ammoniated WS2 the NH4+ ions bound to the layer do not get released easily leading to well solvated layers. The photographs of the dispersions of ammoniated MS2 obtained in water are shown in Figure 6. The dilute black

Figure 6. Photographs of the colloidal dispersions of ammoniated MS2 in water.

transparent colloidal dispersions (Figure 6a,b) suggests exfoliation of MS2 layers and the colloidal nature of the dispersions is confirmed by the Tyndall effect shown by the dispersions (Figure 6a,b). Figure 6c shows the stable opaque colloidal dispersions of higher concentrations of MS2 layers. Low-magnification SEM images of MoS2 and WS2 nanosheets dried on Si substrate (Supporting Information, Figure S3) indicates that the colloidal dispersions of ammoniated MS2 largely consists of nanosheets whose lateral size is in the order of micrometers. In the bright-field TEM image (Figure 7a) of the sample, obtained by drying a drop of the dilute ammoniated MoS2 dispersion in water on copper grid, thin folded, laterally micrometer sized layers are seen. Similar transparent layers are seen in the case of ammoniated WS2 (Figure 7c). The HRTEM images of the MoS2 and WS2 layers (Figure 7b,d) clearly show that the layers are crystalline. Lattice fringes due to the (100) and (101) lattice planes, respectively, are seen in the cases of MoS2 and WS2. The electron diffraction patterns obtained from these layers (insets in Figure 4b and d) could be indexed to hexagonal MoS2 and WS2 respectively. The AFM images of the layers deposited on a Si wafer from dilute colloidal dispersions of ammoniated MS2 in water prove exfoliation (Figure 8). In both, ammoniated MoS2 and WS2, we observe large sheets with lateral dimensions of around 1−2 μm and thickness of about 2 nm. These nanosheets are made of 2− 3 layers of MS2. Exfoliation of ammoniated MS2 yields large

Figure 7. Bright-field TEM images (a,c) and HRTEM images (b,d) of MoS2 and WS2 layers, respectively, obtained through exfoliation of ammoniated MS2 in water.

nanosheets of lateral dimensions in the order of micrometers and this is a definite advantage. The MS2 layers could be restacked from the monolayer colloidal dispersions by evaporating the solvent. The XRD patterns of the solids obtained on restacking of layers from the colloidal dispersions of ammoniated MS2 (Figure 9) suggest that these are less ordered along the stacking direction compared to the precursor MS2 samples (Figure 2). The solid obtained from the dispersion of ammoniated MoS2 in water (Figure 9a) shows a pattern similar to that of pristine MoS2 (Figure 2a) except that the peaks are broadened due to disordered stacking. This suggests that the restacked sample is free from interlayer NH3/NH4+ species. The sample obtained by restacking the layers from octanethiol dispersion is a lot more disordered and it also shows a peak corresponding to the basal spacing observed in the case of ammoniated MoS2 (Figure 2b) in addition to the reflections corresponding to MoS2. Here the intercalated NH3 is not lost as in the case of restacking from water dispersion. This could be attributed to the poor solvation of NH3 by octanethiol that allows these molecules to be bound to the 1391

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Figure 8. AFM images of MoS2 (a,b) and WS2 (c,d) layers obtained from exfoliation of ammoniated MS2 in water.

of N 1s peak (Supporting Information, Figure S4c). In the case of restacked WS2, the presence of N 1s peak corresponding to NH4+ alone suggests that the interlayer has only NH4+ and not NH3. Figure 10 compares the Raman spectra of the MS2 layers obtained on exfoliation of ammoniated MS2 in water and 1octanethiol and the restacked MS2 layers with that of the bulk. In both the cases, MoS2 as well as WS2, increased fwhm and the shift in the A1g and E2g modes for the exfoliated layers compared to those of bulk clearly indicate softening of A1g and E2g modes and phonon confinement that is expected for mono to few layer MS2.30,51−53 This suggests that exfoliation of ammoniated MS2 yields largely mono to few layered MS2. The fact that the Raman spectra of the restacked samples are similar to those of the exfoliated samples suggests that the restacked samples consist of poorly ordered stacking of the MS2 layers leading to a house of cards structure as indicated by their XRD patterns (Figure 9). Synthesis of MoS2−WS2 hybrids is schematically depicted in Figure 11. Coagulation of the suspended MS2 nanosheets by removal of solvent by evaporation results in random stacking of the two MS2 layers. The XRD patterns of MoS2−WS2 hybrids of varying composition are shown in Figure 12, panel A. In comparison to pristine ammoniated MoS2 and WS2 (Figure 12, panel A, a,b), the decreased basal spacing in the case of the MoS2−WS2 hybrids (Figure 12, panel A, c−e) indicates the absence of NH3/NH4+ species in the interlayer. Broad basal and in plane reflections in the XRD pattern of the MoS2−WS2 hybrid suggests that the hybrids are highly disordered.

Figure 9. XRD patterns of MoS2 (a,b) and WS2 (c,d) obtained by restacking the layers from the colloidal dispersions of ammoniated MS2 in 1-octanethiol and water, respectively.

layers. In the case of ammoniated WS2, both aqueous as well as octanethiol dispersions yield very poorly ordered ammoniated WS2 on restacking (Figure 9c,d). This is because the intercalated NH4+ ions do not leave the layers as readily as NH3. The XRD observations are further corroborated by the XPS spectra of the MS2 samples obtained by restacking from their colloidal dispersions in water (Supporting Information, Figure S4). In comparison to the XPS spectra of the original ammoniated MS2 (Figure 4), the XPS spectra of the restacked samples (Supporting Information, Figure S4) indicate that the MS2 phase remains intact in both the cases. The absence of interlayer NH3 in the case of MoS2 is confirmed by the absence 1392

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Figure 10. Raman spectra of MoS2 (panel A) and WS2 (panel B): bulk (a), ammoniated MS2 exfoliated in water (b), and in 1-octanethiol (c); restacked MS2 from the colloidal dispersions in water (d) and in 1-octanethiol (e). Raman shift and the fwhm of the peaks are summarized in the table.

Figure 11. Schematic representation of formation of MoS2−WS2 hybrid.

(Figure 12, panel B, c−e) compared to pristine ammoniated WS2 and MoS2 (Figure 12, panel B a,b) clearly indicate softening of A1g and E2g modes and phonon confinement that is characteristic of mono to few layer MS2.30,51−53 This further confirms that MoS2−WS2 hybrids are composed of poorly stacked MS2 nanosheets. It is also important to note that the MoS2 and WS2 specific intensities follow a trend. The intensities of A1g and E2g modes of MoS2 decreases as the MoS2 composition in the MoS2−WS2 hybrid decreases in going from 75:25 to 25:75 mol ratio of MoS2/WS2 (Figure 12, panel B, c−e). Bright-field TEM image (Figure 13a) of the 50:50 MoS2− WS2 hybrid shows regions of varied contrast indicating stacking of MS2 layers. The diffused rings in the ED pattern (shown as inset in Figure 13a) could be indexed to 101 and 106 planes of MoS2 and WS2 (both have similar lattice parameters). Lattice fringes in HRTEM images (Figure 13b−d) from various regions of the MoS2−WS2 hybrid could be attributed to basal spacing (0.65 nm) as well as in plane lattice (0.24−0.27 nm).

Figure 12. XRD patterns (panel A) and Raman spectra (panel B) of the as prepared ammoniated WS2 (a) and ammoniated MoS2 (b) in comparison to MoS2−WS2 hybrids of varying mole ratios of 75:25 (c), 50:50 (d), and 25:75 (e).

Increased fwhm and the shift in the A1g and E2g modes of the Raman spectra of MoS2−WS2 hybrids of varying compositions 1393

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Figure 14. (A) Photoluminescence spectra of bulk MoS2 and WS2 (a,c); MoS2 and WS2 nanosheets dried on Si substrate (b,d). (B) Photoluminescence spectra of MoS2−WS2 hybrids of varying compositions (mole ratio 75:25, 50:50, and 25:75).

the colloidal dispersion of ammoniated MS2 largely consist of few-layered MS2 nanosheets. PL spectra of the MoS2−WS2 hybrids (Figure 14, panel B) exhibit a peak at 820 nm due to the indirect bandgap and a peak at 705 nm, whose intensity increases with the MoS2 content, along with a weak shoulder at 658 nm. The MoS2−WS2 heterojunctions were predicted to show direct bandgap emission that should be red shifted compared to the A excitonic emission of MoS2/WS2 layers.45 The altered direct bandgap emission further proves the formation of the hybrids that have heterojunctions.

4. CONCLUSIONS NH3 intercalated MoS2 and NH3 and NH4+ intercalated WS2 have been prepared by reacting the corresponding LixMS2 with an aqueous solution of NH4Cl. While the intercalated NH3 gets slowly released from these solids, NH4+ ions do not leave the interlayer even after three months. The ammoniated MS2 exfoliate readily in polar solvents giving colloidal dispersions of MS2 nanosheets of large lateral dimensions. When the layers are restacked from the colloidal dispersions, intercalant-free MoS2 and NH4+ intercalated WS2 are obtained. The layers of MoS2 and WS2 could be costacked to form MoS2−WS2 hybrids in which the nanoplatelets of the two sulfides are randomly stacked together and intimately mixed. The fact that ammoniated MS2 nanosheets are relatively easy to handle compared to LixMS2 and that they exfoliate readily in a number of polar protic solvents to give high yield of large nanosheets make these solids suitable precursors for MS2-based nanohybrids. It should be kept in mind that these solids degrade slowly when stored in ambient conditions and they should be used within a few days after preparation. Ammoniated WS2 is more promising than its Mo analog because the former is relatively more stable. While the MS2 nanosheets exhibit a direct bandgap, the MoS2−WS2 hybrids show a lower direct bandgap energy compared to the precursor MS2 layers.

Figure 13. Bright-field TEM image (a), HRTEM images from different regions (b−d), HAADF STEM image (e), and the spatially resolved S (f), Mo (g), and W (h) elemental maps of MoS2−WS2 hybrid (mole ratio 50:50).

Chemical analysis of the hybrid was carried out using highangle annular dark-field (HAADF) scanning transmission electron microscopy (STEM) and EDS elemental mapping. Figure 13e presents a HAADF STEM image of the MoS2−WS2 hybrid. The elemental maps of the constituting elements S, Mo, and W (Figure 13f−h) clearly demonstrate a well-defined compositional profile of the MoS2−WS2 hybrid, suggesting that the hybrid is a uniform mixture of both MoS2 and WS2 layers. Uniform composition was further confirmed by EDS analysis from several regions of the hybrid. Bulk MoS2 and WS2 (Figure 14, panel A, a,c) exhibit an indirect band gap emission at ∼820 nm. MoS2 nanosheets (Figure 14, panel A, b) exhibit a single emission at ∼670 nm corresponding to A exciton. WS2 nanosheets (Figure 14, panel A, d) exhibit two distinct emissions at ∼670 and ∼615 nm corresponding to A and B excitons. These observations are in accordance with what has been observed in the literature38,53,54 for few-layered MoS2 and WS2 nanosheets, thus suggesting that

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ASSOCIATED CONTENT

S Supporting Information *

SEM images and XPS data of ammoniated MS2. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: mikerajamathi@rediffmail.com. 1394

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Notes

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The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was funded by DST, New Delhi, India (SR/S1/PC23/2011). REFERENCES

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