Article pubs.acs.org/cm
Mechanism for Liquid Phase Exfoliation of MoS2 Ali Jawaid, Dhriti Nepal, Kyoungweon Park, Michael Jespersen, Anthony Qualley, Peter Mirau, Lawrence F. Drummy, and Richard A. Vaia* Materials and Manufacturing Directorate, Air Force Research Laboratory, Wright-Patterson AFB, Ohio 45433-7702, United States S Supporting Information *
ABSTRACT: A highly efficient, reproducible, and scalable approach for exfoliation of MoS2 is critical for utilizing these emerging materials from coatings and composites to printable devices. Additive-free techniques, such as solvent-assisted exfoliation via sonication, are considered to be the most viable approach, where N-methyl-2-pyrrolidone (NMP) is the most effective solvent. However, understanding the mechanism of exfoliation and the key role NMP plays during the process have been elusive and challenges effective improvements in product yield and quality. Here, we report systematic experiments to understand the mechanism of solvent-assisted exfoliation by elucidating the sonolysis chemistries associated with NMP. It is confirmed that in the presence of O2(g) dissolved moisture in NMP plays a critical role during sonication. The higher the moisture content, the more efficient the exfoliation process is. Conversely, when exfoliations are carried out with dried solvents with an inert atmosphere, reaction yields decrease. This is due to redox-active species formed in situ through an autoxidation pathway that converts NMP to N-methyl succinimide by hydroperoxide intermediates. These highly reactive species appear to aid exfoliation by oxidation at reactive edge sites; the charging creates Coulombic repulsion between neighboring sheets that disrupts interlayer basal plane bonding and enables electrostatic stabilization of particles in high-dipole solvents. From these insights, exfoliation in previously reported inactive solvents (e.g., acetonitrile), as well as in the absence of probe sonication, is demonstrated. These findings illustrate that exfoliation of MoS2, and possibly TMD’s in general, can be mediated through understanding the chemistry occurring at the surface−solvent interface.
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INTRODUCTION Atomically thin, two-dimensional layered materials, such as graphene and transition metal dichalcogenides (TMDs), have attracted significant interest because of their size-tunable electronic,1,2 optical,3,4 and catalytic5,6 properties. In the case of TMDs, the presence of unsaturated d-orbitals and chemically active edge sites results in a broad diversity of materials, including catalysts, semiconductors, semimetals, ferromagnets, and even superconductors.7 As with graphene, single and fewlayer TMDs can be grown from precursors8−10 or exfoliated from bulk crystals.11,12 The latter approach is crucial for utilizing this broad property space in large-scale applications including composites, coatings and inks for additive manufacturing. However, a limited understanding of the mechanisms underlying TMD exfoliation limits the development of practical methods to eliminate processing-induced defects and target site-specific surface functionalization and to cost-effectively scale high-yield manufacturing of few-layer TMDs. Molybdenum disulfide (MoS2) is one of the most commonly researched TMDs due to its size-dependent optical and electronic properties. The MoS2 crystal structure consists of 0.65 nm thick layers of Mo atoms coordinated between two sulfide planes, which are stacked and separated by a van der Waals gap. At the few- to monolayer limit, an indirect-to-direct band gap transition occurs due to thickness-induced quantum © 2015 American Chemical Society
confinement. This leads to photoluminescence at the band edge (1.8 eV), which can be exploited for chemical sensing and photocatalysis. Additionally, this thickness-dependent transition also induces strong spin−orbit coupling, which can lead to advanced device applications in nanoelectronics, sensors, and spintronics.13,14 Three general approaches (micromechanical, ion-intercalation, and liquid phase) have been developed to disrupt the weak van der Waals attraction between sheets and form few- to monolayer MoS2 from bulk sources. Micromechanical cleavage utilizes adhesive tape peeling or powder grinding to overcome layer−layer attraction with shear force. The yield of high-quality monolayers from this approach is low; sufficient material can be obtained for laboratory experiments, but scale-up is challenging.15,16 Alternatively, ion-intercalation disrupts interlayer attraction through layer reduction and concomitant insertion of exchangeable ions into the interlayer gap.17,18 For example, n-butyl lithium reduces MoS2, resulting in a lateral crystallographic expansion upon stoichiometric intercalation of lithium ions (1:1 Li/Mo).19,20 Subsequent agitation or pH adjustment disperses individual sheets, which subsequently can be restored to their original morphology by Received: November 2, 2015 Revised: December 16, 2015 Published: December 17, 2015 337
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Chemistry of Materials acid wash to liberate the intercalated ions.19 This method has gained popularity due to its selectivity to form monolayer MoS2 in common organic solvents (e.g., THF).21 However, the need for strong intercalating agents (i.e., n-butyl lithium, sodium napthalenide), inert atmospheres, long reaction times (>48 h), and postexfoliation steps to restore the MoS2 structure presents challenges for safe, effective scale-up. Recently, the concept of liquid phase exfoliation22 has been developed to circumvent the limitations of these other approaches by using large loading ratios and ambient environments and reducing the use of hazardous chemicals. As an example, high-concentration dispersions (up to 30 mg/ mL) of few- to monolayer MoS2 can be obtained in N-methyl2-pyrrolidone (NMP) in the absence of surfactants through the use of a carefully optimized solvent and aggressive, prolonged mechanical agitation.23 Fundamentally, it is speculated that surface tension of a good solvent should closely match the surface energy of the TMD as the entropic contribution from exfoliation of rigid, unfunctionalized sheets is negligible.24 For MoS2, the surface tension of NMP (40 mJ/m2) is similar to estimated surface energies of few-layered MoS2 (46.5 mJ/m2).25 Mechanical agitation via probe sonication in NMP is sufficient, then, to overcome the van der Waals interactions between layers and suspend the TMDs in the solvent. While this framework seems to help evaluate potential solvents to form stable dispersions,26 the exfoliation mechanism and potential role of chemical processes remains elusive. For example, this framework account for neither the origin of a negative charge reported on dispersed MoS2 sheets27,28 nor its subsequent role in stabilizing the colloidal dispersion in high-dipole organic solvents. Also, recent studies have shown that previously good solvents become poor solvents if an initial mechanical grinding step is added to the process.29 To further our understanding of the mechanism of TMD exfoliation, herein we discuss the chemical aspects of the liquid phase exfoliation of MoS2 in NMP and other high dielectric solvents. A systematic study of the stability of NMP during probe sonication demonstrates that NMP undergoes autoxidation that, in the presence of O2 and H2O, results in the formation of hydroperoxides. These active species oxidize MoS2 and facilitate exfoliation. This makes hydrous NMP an excellent solvent for ultrasonic-assisted exfoliation due to its tendency to form these reactive peroxides, whereas anhydrous NMP in the absence of O2 is a poor solvent. Therefore, alternative processing routes that enhance NMP autoxidation, such as mild heating (70−100 °C), yields MoS2 exfoliation in the absence of probe sonication. The byproducts of MoS2 oxidation are consistent with molybdenum-blue anionic clusters and oligomers, which likely adsorb on the MoS2 surfaces and impart a high negative surface charge. By following this mechanism, facile exfoliation of MoS2 can be transferred to other previously reported nonactive solvents, such as acetonitrile, through the intentional addition of small amounts of species isolated from NMP autoxidation.
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DMF and NMP were distilled under calcium hydride and stored under activated molecular sieves in argon-purged bottles. Methods. Distillation of NMP Degradation Products. Ten milliliters of NMP and 1 mL of H2O were added into a flat-bottomed scintillation vial and mixed until the solution was homogeneous. The solution was then sonicated for 2 h with a 7 s on and 5 s off pulse at a power output of 127.5 W. Temperature was maintained at 25 °C by use of a water-cooled bath. As noted below, these conditions duplicate those used for MoS2 exfoliation. The resultant optically clear yellow solution was transferred to a 25 mL round-bottomed flask and distilled under vacuum (200 mTorr). Three fractions were observed. The first was removed at room temperature and is attributed to H2O. The second fraction was removed at 70 °C with a volume of 9.5 mL. This fraction was attributed to NMP. The remaining liquid was an optically transparent yellow liquid and was pipetted out (ca. 500 μL) and analyzed by FTIR, NMR, and GC-MS. GC-MS analysis was performed after functionalization with MTBSTFA (100:1 v/v MTBSTFA/ distillate) and diluting in anhydrous ACN to a final concentration of 100 mg/L without further purification. MoS2 Exfoliation by Sonication. Samples were prepared by combining the additives (methyl succinimide, succinimide, and/or water) with 10 mL of solvent in a 20 mL flat vial. Exfoliations using anhydrous solvents were performed using freshly distilled solvents with the addition of activated molecular sieves to the vessel. Exfoliations under an inert atmosphere were performed with continuous flushing of the reaction vial with Ar gas 60 min prior to exfoliation and throughout the exfoliation process. After the solution was homogenized, 30 mg of MoS2 powder was added. Samples were sonicated (Sonics and Materials Inc.) continuously for 60 min using a horn probe sonic tip (5 mm, tapered microtip 630-0419 vibra cell) at a power output of 127.5 W (17% × 750 W) in a water-cooled bath at 25 °C. The tip was pulsed for 7 s on and 5 s off to avoid excessive heating. After sonication, the dispersions were centrifuged for 10 min at decreasing speeds between 12 700 and 2000 rpm, where approximately the top 75% of the supernatant was collected by pipet for analysis at each speed. The pelleted MoS2 was resuspended in additional solvent and centrifuged again at a lower centrifugal speed (i.e., 10 000 rpm) to isolate the next fraction of flakes. In this manner, sequential centrifugation and resuspension allows for separation of sheets by increasing size. Sizes isolated were from 12 700, 10 000, 7000, and 5000 rpm. Probeless MoS2 Exfoliation. Into a three-necked round-bottomed flask equipped with a stir bar and reflux condenser were added 10 mL of solvent and 100 mg of MoS2. To this suspension was added NMS, water, or a combination of both, and the samples were heated to 100 °C for 1 h to promote formation of hydroperoxides. When processing under ambient conditions, the vessel was purged with air throughout the reaction. Inert conditions were prepared by drying the solvent under vacuum (100 mTorr) at 40 °C for 1 h, followed by backfilling with argon gas. This procedure was performed three times to ensure the removal of O2 and H2O. After the reaction, the vessel was allowed to cool to room temperature after removing from the heat source. After cooling, a 3 mL aliquot was taken and centrifuged at 5000 rpm for 5 min. The discolored supernatant was discarded, and sediment containing MoS2 was subsequently redispersed in fresh solvent and homogenized in a bath sonicator (Branson 1200) for 120 s. This washing step was necessary to disperse exfoliated sheets due to degradation of NMP and DMF at elevated temperatures, which inhibits flakes from entering the liquid phase. The final dispersion was again centrifuged at 5000 rpm for 10 min, and the supernatant containing exfoliated flakes was collected by pipet and analyzed by UV−vis spectroscopy. Note that higher MoS2/solvent ratios were used for probeless exfoliation, as we observed less exfoliation compared to that with probe sonication techniques. We attribute this to unoptimized reaction conditions for hydroperoxide formation. Nevertheless, the relative effect of different additives under probeless sonication can be understood by increasing the initial amount of MoS2 to afford visual confirmation as well as clear differences in the UV−vis characterization.
EXPERIMENTAL SECTION
Materials. NMP, N-methyl succinimide (NMS), dimethylformamide (DMF), acetonitrile (ACN), molybdenum(IV) sulfide (lot no. 234842), and N-tert-butyldimethylsilyl-N-methyltrifluoroacetamide (MTBSTFA) were purchased from Sigma-Aldrich. Succinimide and calcium hydride were purchased from Fluka. Deionized water was obtained from a Milli-Q solvent system with a resistance of 18 MΩ. 338
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Chemistry of Materials Characterization. Determination of MoS2 Concentrations. The absorption cross-section of exfoliated flakes was estimated using methods developed by O’Niell et al.23 The mass of flakes in the parent suspension was determined, followed by the collection of UV−vis spectra from a dilution series. Low centrifugal rates (2000 rpm) were employed to harvest a high concentration of exfoliated flakes in the supernatant. UV−vis absorption was then carried out on a series of dilutions (up to 100×). Higher centrifugal rates did not yield an adequate concentration to enable the creation of a sufficiently large dilution series. Due to the large size of the flakes collected at 2000 rpm, however, a large scattering background was superimposed on the characteristic resonances at ∼680 nm (Figure S1a).30 The contribution from scattering to this resonant absorption was removed using a linear scattering estimate extrapolated from high wavelengths, as shown in Figure S1b. The concentration from each absorption measurement was determined by drying a known volume of exfoliated flakes under vacuum (120 mTorr, 100 °C) and taking the resultant mass difference. A linear relationship between concentration, C, and resonant absorption, Ar, gave a molar absorption cross-section of ε = 1200 mL/mg/m (Figure S1c, Ar/l = εC). This is similar to and bounded by previously reported values.23,31−33 Note that the absorption crosssection is invariant to flake size, allowing concentrations to be determined regardless of the degree of scattering.34 Due to the heterogeneity in particle size and purity within a MoS2 mineral powder, as well as variability between suppliers, there is not an accepted method to express theoretical or absolute yield of exfoliation as a means to ascertain efficiency of different reaction or processing methods. Additionally, a yield with respect to suspended flakes is not equivalent to a yield with respect to exfoliated flakes; the former includes particles that do not sediment, whereas the latter is a subset of particles with one layer to a few layers. Thus, we adopt a definition of yield that is self-referenced with a common postprocessing procedure (12 000 rpm, 10 min). High centrifugation speeds were chosen to isolate exfoliated materials while simultaneously separating larger unexfoliated (yet suspended) particles. With this approach, absorbance data is directly proportional to the concentration (mass/volume ratio) of exfoliated material, allowing for a direct comparison to be made between reaction conditions, including modifications to the traditional liquid phase exfoliation procedure. Thus, relative yields of different methods are reported herein as a concentration after a common postprocessing procedure. Spectroscopic monitoring of exfoliation was facilitated by the use of MoS2 sheets with small lateral dimensions where low, nonresonant scattering gave well-resolved excitonic features. Lateral polydispersity was minimized by the use of high centrifugal speeds (12 700 rpm) to sediment and remove larger flakes and short sonication times (1 h) to minimize scission-induced variance of lateral dimensions.23 Although these short process times limit the overall yield from sonication (micromolar concentrations), they enabled the relative comparison of the effectiveness of chemical processes to drive exfoliation. Spectroscopy. UV−vis spectra of exfoliated samples were measured on a Cary 300 spectrometer. All obtained UV−vis spectra were corrected for scattering by a linear scatter correction at nonresonant wavelengths (850−800 nm). Microscopy. Transmission electron microscope (TEM) images were collected on an FEI Talos TEM with an accelerating voltage of 200 kV. STEM images and corresponding energy-dispersive X-ray (EDS) spectra were obtained probing the L edge of molybdenum and K edges of oxygen and sulfur. Scanning electron microscope (SEM) images were collected on a Hitachi S5200 SEM at an accelerating voltage of 30 kV and beam current of 10 μA. The samples were prepared by spotting a Formvar-coated copper grid with 10 μL of exfoliated MoS2 and drying overnight under benchtop conditions. Statistical analysis of 5000 and 12 000 rpm centrifugation products confirms size selection from 400 ± 150 nm for flakes isolated at 12 000 rpm and 1.5 μm ± 500 nm for flakes isolated at 5000 rpm. Gas Chromatography Mass Spectroscopy (GC-MS). To increase volatility of the analytes, MTBSTFA functionalization was utilized, and MTBSTFA and NMP blanks were used to correctly identify peaks corresponding to the derivatized distillate. Samples were dissolved into
MTBSTFA and diluted with a minimal amount of ACN to a concentration of 100 mg/L. GC separation of derivatized compounds was carried out on a Thermo Trace Ultra GC equipped with a 15 m long Thermo TG-SQC column. The column internal diameter was 0.25 mm with a film thickness of 0.25 μm. Analysis was performed with a TSQ Quantum XLS triple quadrupole mass spectrometer operated in full scan mode using Q1. The scanning range was 35−550 AMU with a cycle time of 0.25 s. One microliter of sample was injected into a splitless inlet maintained at 250 °C, where the helium carrier gas was maintained at a constant flow rate of 1.2 mL/min. Oven temperature was initially held at 45 °C for 3 min and then ramped to 320 °C at a rate of 15 °C/min and held for 2 min. X-ray Photoelectron Spectroscopy (XPS). Native silicon oxide wafers (∼300 nm) were cut into ∼1 cm2 pieces and cleaned by dipping into an isopropanol bath, followed by sonication for 5 min. This was followed by rinsing in acetone and isopropanol and drying with N2. These dried and cleaned wafers were kept under UV−ozone for 20 min to activate the oxide surface. Freshly exfoliated MoS2 flakes were first centrifuged at 2000 rpm for 30 min to remove unexfoliated material. The supernatant was carefully removed and centrifuged at 12 000, 10 000, and 7000 rpm to isolate flakes with increasing lateral dimensions as discussed above. Five microliters of the supernatant was pipetted out and drop cast onto clean Si wafers. Finally, these wafers were dried under vacuum in an oven at 3 × 10−6 Torr at 100 °C for 18 h. Note that it was found that increasing the temperature beyond a critical value (ca. 100 °C) under vacuum results in removal of the oxides present on the surface and is not reflective of sonicationinduced oxidation; thus, low temperatures were used to remove residual solvent and prevent thermal annealing. XPS analysis was carried out using a Kratos AXIS Ultra spectrometer under highvacuum conditions (∼2 × 10−9 Torr). Survey spectra were acquired using a monochromated Al Kα X-ray source (1486.6 eV) operated at 120 W (10 mA, 12 kV), with the electron analyzer operating in hybrid lens mode and an aperture size of approximately 300 μm × 700 μm. Survey data were acquired at an analyzer pass energy of 160 eV, using 1 eV steps and a dwell time of 400 ms. High-resolution data of the Mo 3d, S 2p, O 1s, and C 1s regions was collected using an analyzer pass energy of 20 eV and a step size of 0.1 eV. Data analysis was carried out using the CasaXPS software package. Peak areas were determined using a Shirley background subtraction, and atomic concentrations were calculated by applying relative sensitivity factors for the XPS instrument. Curve fits for the Mo 3d and S 2s regions were obtained by fitting the peaks with a Gaussian−Lorentzian function with a 70% Lorentz contribution.
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RESULTS AND DISCUSSION Over the course of our examination of MoS2 exfoliation in NMP, we noted that the yield varied considerably based on the supplier, age, and storage conditions of NMP. We observed that older bottles of NMP consistently resulted in higher yields, whereas anhydrous solvent resulted in the poorest yields and older NMP often exhibited a slight yellow discoloration. Many lactams such as NMP are hygroscopic and have rich chemistries associated with nucleophilic addition, elimination, and ring opening.35 These factors indicate that chemical species produced via NMP degradation, whether during storage or as a consequence of processing, may influence exfoliation yields. To summarize these observations, Figure 1 compares the absorption spectra of exfoliated MoS2 in NMP (12 700 rpm) as microquantities of water are added to an initially anhydrous solvent. Note that the initial amount of water added is less than 1 vol% (1:200 mol/mol H2O/NMP); thus, careful attention was paid to ensure that the water content in the anhydrous NMP was the absolute minimum by additional vacuum distillation under calcium hydride and storage under molecular sieves. In general, increasing the amount of water in NMP increases yields from 5 to 30 μg/mL for 12 000 rpm centrifugal 339
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Figure 2. Sonication-mediated peroxidation of NMP in the presence of water. (A) Increased discoloration of NMP/H2O mixtures after probe sonication (1 h, 25 °C) with the indicated amounts of water added prior to sonication. 0 mmol H2O corresponds to NMP distilled and stored under molecular sieves. (B) Proposed scheme for the decomposition of NMP via free radical-promoted autoxidation from work performed by Anderson and Drago.36,39 The hydroperoxide is likely the active compound promoting high exfoliation yields in NMP. Decomposition of the peroxide intermediate generates 5-hydroxy-Nmethyl-2-pyrrolidone (5hmp) and further oxidation products such as N-methyl-succinimide (NMS).
Figure 1. Impact of microaddition of water on the exfoliation of MoS2 in NMP. (A) Photograph of increasing exfoliation of MoS2 flakes as water (0−55 mmol) is added to anhydrous NMP (leftmost). (B) Corresponding absorption spectra of exfoliated MoS2 solutions. Samples were sonicated (3 mg/mL MoS2 in NMP) for 1 h with the indicated amounts of H2O. Subsequent centrifugation at 12 700 rpm resulted in transparent suspensions of exfoliated MoS2 in the supernatant.
fractions, as shown in Figure 1B. Similar behavior is observed for all fractions, irrespective of centrifugation speed, indicating that the impact of water is pervasive and not restricted to the smallest flakes (Figure S2). Autoxidation of NMP has been studied for more than 30 years; for example, oxidative products are present in NMP recovery after high-temperature coal extraction. Under reflux conditions, the presence of O2 facilitates free radical production after α-hydrogen abstraction and possibly N-methyl abstraction in the presence of H2O. This initiates formation of oxidative products including 5-hydroxy-N-methyl-2-pyrrolidone,36,37 2pyrrolidone,38 and NMS36,39 via autoxidation-initiated hydroperoxide38,36 intermediates, as shown in Figure 2. Furthermore, decomposition occurs at temperatures as low as 85 °C in the presence of O2 and H2O, as shown by the presence of NMS, the major product in this autoxidative process.39 Removal of O2 and H2O during these processes reduces the amount of these degradation products.39 Overall, the oxidation of NMP forms reactive α- and methyl- radicals,40,41 leading to N- and ωhydroperoxides when exposed to O2 and H2O for prolonged periods of time or at elevated temperatures.36,38 The hygroscopic nature of NMP and conditions created by probe sonication, including high local temperature associated with cavitation of bubbles and active mixing with the atmosphere, produce ideal conditions for autoxidative transformations of NMP,42 as is often reflected in NMP discoloration. Figure 2A shows the development of such a yellow discoloration of NMP as the fraction of water or sonication time is increased in the absence of MoS2. The in situ generated species in oxidized NMP was isolated via vacuum distillation and further characterized. The experiments showed that approximately 5 vol % of a new yellow species was formed after 120 min of sonication of 25 mol %
H2O/NMP in samples where neither NMP nor H2O was deoxygenated. FTIR spectra of the distillate showed the presence of aliphatic hydroxyl peaks absent in the anhydrous solvent. The free hydroxyl peak present in the dry NMP was absent in the isolated distillate (Figure S3). 1H and 13C NMR indicate the emergence of hydroxylated carbons, secondary amides, and carboxylic acids, suggesting that multiple chemical pathways are active during sonication (Figure S4). Figure 3 summarizes GC chromatograms of the distillate after functionalization with MTBSTFA to increase volatility. The dominant product at 10.68 s (Figure 3b) gave a mass spectrogram consistent with 5-hydroxy-N-methyl-2-pyrrolidone (5hmp; Figure 3a). Identification of products with longer retention times (10.84 s) was elusive but may correspond to hydroxyl loss of unfunctionalized 5hmp (MW 99 g/mol) or residual NMP. Additional species eluted at 10.45 and 10.50 s, corresponding to products less polar than 5hmp, suggesting that NMS is a minor product. The presence of 5hmp strongly suggests that the autoxidative pathway discussed above (Figure 2B) is active during sonication. High local temperatures in excess of 1000 K43,44 induced by probe sonication may form radical species at the γ position in NMP. Subsequent uptake of O2 in the presence of H2O forms relatively stable hydroperoxides, which then oxidize NMP to form 5-hydroxy-N-methyl-2-pyrrolidone, a pathway that has been shown to be redox-active for nanoparticle formation.37 The 5hmp alcohol can undergo further oxidation to form NMS via bimolecular elimination promoted by H2O. Because of the relatively low fractions of other oxidative 340
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Figure 3. Chromatogram and GC-MS analysis (inset) of oxidative products formed after sonication of 10 mL of NMP and 55 mmol of H2O. The yellow oxidative product was isolated by vacuum distillation (50 °C, 100 mTorr) after removal of H2O (25 °C) and NMP (40 °C) and subsequently functionalized with MTBSTFA. (A) Standard. Methyl tert-butyl silyl ether (MTBSTFA)-derivatized 5-hydroxy N-methyl pyrrolidone (5hmp). (B) Isolated oxidized product. The major product corresponds to 5hmp (10.68 s), consistent with the autoxidation process outlined in Figure 2B. Lower retention time products (10.4−10.5 s) likely correspond to N-methyl-succinimide (NMS) or residual NMP. Species at 10.84 s was not conclusively identified, but it likely corresponds to residual NMP and unfunctionalized 5hmp.
Figure 4. UV−vis spectra of exfoliated MoS2 in ACN (left) and DMF (right) with succinimide (Suc.) and N-methyl succinimide (NMS) additives. Exfoliation in anhydrous ACN with increasing amounts of (A) succinimide and (B) NMS. Exfoliation in anhydrous DMF with increasing amounts of (C) succinimide and (D) NMS in the presence and absence of H2O. Optical images are provided for (A) and (C) above their respective spectra.
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Chemistry of Materials Table 1. Concentration of MoS2 Exfoliationa in NMP, DMF, and ACNa atmosphere
inert
ambient
inert
ambient
ambient
ambient
ambient
ambient
additive (mmol)
0
0
0
0
Suc. (5)
Suc. (5)
NMS (5)
NMS (5)
H2O (mmol)
0
0
50
50
0
5
0
5
NMP (μg/mL) DMF (μg/mL) ACN (μg/mL)
2.5 3.5 N/A
5.0 5.5 1.5
6.8 N/A N/A
24.2 3.0 N/A
10.0 7.5 N/A
12.5 9.8 12.2
11.2 9.0 5.0
33.5 10.1 20.5
a
Processing: 1 h sonication, 12 700 rpm. Concentration based on UV−vis analysis. Values reported are in micrograms per milliliter. N/A corresponds to no exfoliation.
Figure 5. STEM, HRTEM, and EDS characterization of a MoS2 flakes. (A) SAED of flakes resolves multiple crystallographic planes that correspond to single-crystalline MoS2. (B) HRTEM images confirm that the isolated flakes are single crystalline (2 nm scale bar), and (C) representative HRTEM of a multilayered flake exhibits Moiré patterns from rotated stacked crystallographic planes (10 nm scale bar). (D) STEM image and elemental mapping of S, Mo, and O (20 nm scale bars).
products, such as NMS, it is likely that formation of NMS via 5hmp is reversible45 under sonolytic conditions. Overall, these results indicate that NMP is not a static solvent during probe sonication and that its reactivity plays a role in the high exfoliation yields of MoS2. Key to this process is the presence of O2 and H2O. It has been shown that substrates similar to NMP can form radicals and hydroperoxides, such as 1,5-dimethyl-2-pyrrolidone and L-prolinamide;36 thus, addition of species from the oxidative pathway to anhydrous solvents should similarly increase yields. To demonstrate this, Figure 4 and Table 1 compare MoS2 exfoliation yields in NMP to anhydrous DMF and ACN for the same processing conditions and the addition of NMS, succinimide, and H2O under inert (argon) and oxidative (ambient) atmospheres. Unless noted, samples were exfoliated under an ambient atmosphere. All solvents were anhydrous and prepared by distilling and storing under molecular sieves. Note that complete deoxygenation of all solvents was not achieved and that use of an inert
atmosphere (Ar purge) minimizes the additional incorporation of O2 and H2O during sonication. In general, exfoliation in DMF and ACN dramatically increased with small additions (1−5 mmol) of NMS or succinimide. The impact of these additives is greatest under an ambient atmosphere with coaddition of water. Figure 4A and Figure 4B, for example, demonstrate the effect of additions of NMS and succinimide to anhydrous ACN. We observed that anhydrous acetonitrile under an ambient atmosphere failed to exfoliate detectable amounts of flakes, whereas the addition of 5 mmol of succinimide or NMS with coaddition of 25 mmol of H2O increased yields up to 12 and 20 μg/mL, respectively. For comparison, anhydrous NMP exfoliation under ambient yields approximately 4 μg/mL, and addition of 50 mmol of H2O to NMP provides exfoliation of 24−25 μg/mL. Similar trends are observed with addition of succinimide and NMS to DMF under an ambient atmosphere (Figures 4C,D). Yields from anhydrous DMF increased from 4 to 7 μg/mL for 5 mmol of succinimide and to 9 μg/mL for 5 mmol of NMS. Coaddition of H2O with 342
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Figure 6. XPS spectra of the surface of films prepared from exfoliated MoS2 as water content increases in NMP (left to right). Mean flake size decreases in supernatant as centrifugation speed increases (top to bottom). The above lines indicate the energy level of Mo(VI) 3d5/2 and Mo(IV) 3d5/2 at 236.1 and 229.1 eV, respectively. For a given water fraction, as centrifugal speed is increased, the ratio of Mo6+/Mo4+ increases (Table S1). For a constant centrifugal speed, increasing the water fraction also increases the Mo6+ population. Overall, the S/Mo(IV) ratio is relatively constant (Table S1).
and DMF does not increase exfoliation yields, indicating that the proper substrate is required. For reasonable yields, the autoxidative pathway must be accessible. Processing under an Ar purge lowers the concentration of O2 in the solvents, and addition of up to 50 mmol of H2O to NMP under argon resulted in only a marginal increase in yield relative to that with anhydrous NMP under argon (5 to ∼7 μg/mL), whereas the addition of the same amount of H2O under an ambient atmosphere increased yields substantially (20 μg/mL). A similar result is seen for the other anhydrous solvents where argon purging and reduction of O2 reduces yield. Finally, addition of other products from NMP’s autoxidation, such as N-methyl succinamic acid, failed to increase reaction yields in any solvent.41 N-Methyl succinamic acid evolves from a ring opening nucleophilic substantiation of H2O to NMS, which is
these substrates further increased the yields to approximately 10 μg/mL, an increase of approximately 2.5× from that with anhydrous DMF. Table 1 summarizes exfoliation yields for NMP, ACN, and DMF based on systematic variation of NMS, succinimide, and H2O addition, as well as ambient and inert (argon) processing environments. Samples were sonicated for the same duration (60 min) and centrifuged at 12 700 rpm for 10 min. Inert conditions were prepared by using anhydrous solvent and purging with argon gas 60 min prior to probe sonication and during the sonication process. In general, the data is consistent with an increase in exfoliation under conditions that promote autoxidation of NMP and NMP-like substrates and a reduction in yield under inert conditions, which suppress these chemical transformations. Most notably, addition of only H2O to ACN 343
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Chemistry of Materials considered to be a speculative product in NMP autoxidation.46 This implies that equilibrium processes in which NMS is converted to surface reactive products are not accessible in the ring-opened form. Questions remain, however, about the nature of the chemistry occurring between these oxidative species and the MoS2 surface. It is important to note that the NMP (e.g., water and dissolved O 2 content) and processing conditions (sonication time, strength, and atmosphere) discussed above are well within those commonly reported for liquid phase exfoliation of MoS2 and TMDs. In general, the aforementioned UV−vis spectroscopy of the supernatants (Figures 1 and S1) is consistent with a dispersion of MoS2 flakes. The absorption features at 1.85 eV (680 nm) and 1.98 eV (620 nm) arise from a shift from indirect to direct excitonic transitions at the monolayer limit with the small split arising from spin−orbital coupling,3,47 as highlighted in recent theoretical and experimental papers. The SEM of bulk flakes (nonsonicated, Figure S5a) shows micron-sized crystals, whereas after probe sonication, SEM (Figure S5b) and TEM (Figure S5c,d) of the MoS2 product show ultrathin, sub-200 nm particles irrespective of the processing conditions (i.e., sonication time, atmosphere, addition of H2O, addition of NMS, or centrifugation speed). Selected-area diffractogram clearly resolves the [100], [111], [210], and [300] crystallographic axes of MoS2 (Figure 5A). HRTEM (Figure 5B) confirms the presence of singlecrystalline monolayer MoS2 as well as Moiré fringes (Figure 5C) arising from few-layer MoS2 also present in these samples. High-resolution Raman spectra (Figure S6) resolve only the characteristic E12g and A1g vibrational modes of MoS2, and survey scans on the exfoliated flakes show no additional peaks regardless of exfoliation conditions. However, EDS (Figure 5d) reveals the presence of O along with Mo and S throughout the layer. Stoichiometric quantification of EDS, though, is not possible due to the overlap of the L and K edges of Mo and S, respectively. Quantitative comparison of the surface composition of MoS2 flakes was investigated by XPS (Figure 6). Details of film preparation from exfoliated MoS2 are included in the Experimental Section. As the water content was increased in NMP, the relative ratio of oxidized Mo6+ (3d5/2 at 236.1 eV) to native Mo4+ (Mo 3d5/2 at 229.1 eV) increases, as reflected by the relative intensity of these peaks (Table S1). We also note that the relative abundance of Mo6+ increased as the mean particle size in the supernatant decreased (i.e., as centrifugal speed increased) and the overall S/Mo ratio decreased. For example, at 2.75 mmol of H2O, approximately 50% of the native Mo4+ on the surface is oxidized to Mo6+ in the smallest flakes (12 700 rpm). This corresponds to a decrease in the stoichiometric ratio of S/Mo from 2:1 to 0.52:1. Similar trends are observed when introducing NMS to the exfoliation mixture (Figure S7). Notionally, the increase of Mo6+ with decreasing particle size is reasonable, as edge sites are known to be more chemically active as compared to the relatively inert basal plane.48 Thus, smaller flakes, having a larger fraction of edge sites than larger sheets, would tend to oxidize more readily. However, edge site oxidation cannot fully account for the high abundance of Mo6+ or the shift in the S/Mo ratio. If we assume chemically independent, defect-free sheets (i.e., monolayer limit) for a given flake size, then the volume of basal atoms can be approximated by considering a plate with radius, r, with a monolayer thickness of 0.65 nm. The oxidizable
radius can be approximated as 2× the Mo−S bond length of 0.24 nm,49,50 as it has been shown that Mo edge states penetrate beyond the outermost atoms.51 From the plate radius, the volume of total atoms on the basal surface can be approximated as π(r − 0.48)2 × 0.65 nm, whereas the total volume of the plate can be approximated as πr2 × 0.65 nm. Thus, the fraction of basal atoms to the total volume of the plate can be expressed as ((r − 0.48 nm)2/r2), with the remaining fraction arising from edge atoms, 1 − ((r − 0.48 nm)2/r2). For small plates (i.e., 50.0 nm), the fraction of basal atoms is 0.98, resulting in an atomic edge fraction of 2%; therefore, edge atom oxidation cannot fully explain the observed high Mo6+ percentages. For edge sites to fully account for the observed oxidation (i.e., ∼40%), the mean particle radius would have to be 4−5 nm, sizes not observed in any TEM or SEM micrographs, even for high rpm fractions. In concert with the S/Mo composition changes, though, the specific S/Mo4+ ratio remains approximately constant (∼1.65) for all samples (Table S1). Because the XPS signal is highly surface sensitive, the combination of Mo6+/Mo4+ and S/Mo4+ behavior could alternatively reflect the absorption of a Mo6+ species on the MoS2 flake rather than uniform oxidation of the layer accompanied by loss of sulfur. The formation of a surfaceactive species arising from oxidant−MoS2 reactivity would also clarify the apparent contradiction between bulk spectroscopic measurements (UV−vis and Raman) that indicate the predominance of MoS2, HRTEM that confirm the retention of crystallographic MoS2, and XPS and EDS that imply extensive oxidation and loss of S from the MoS2 sheets. From this characterization of the dispersed MoS2 and previously reported chemistry of MoS2, we propose that liquid phase exfoliation of MoS2 mechanistically consists of two steps: MoS2 oxidation and partial dissolution followed by electrostatic layer−layer repulsion due to the creation of charged defects and absorption of Mo−S−O clusters. First, the presence of oxidants in the exfoliation solution likely reacts preferentially with the edge sites of MoS2 to form MoOx−2 anions with the concomitant etching of the sheet and reduction of lateral dimensions.52 The reactivity of MoS2 edge sites is well-known, with a high selectivity for hydrodesulfurization, C−O bond scission, and hydrodeoxygenation, and contrasts with the chemical inertness of the saturated basal plane. The MoS2 terminal edge contains Mo atoms that occupy a doubly unsaturated coordination sphere, likely facilitating oxidation to Mo6+ and subsequent uptake of oxygen to form MoOx−2 species. The unsaturated sulfur atoms may also be oxidized by hydroperoxides at the layer edge. In the 2H-MoS2 polytype, the partial oxidation of the two unsaturated sulfur atoms has been reported to produce sulfur vacancies.53 The products of this oxidation are likely to yield MoOx−2 and Mo(SOx−2)2 species in solution,52 as shown by the dissolution of exfoliated MoS2 flakes in ethanol to produce highly anionic molybdates.54 Similar observations have been reported during ball milling procedures.55 When scaling up exfoliation solutions (>20 mg/mL) under oxidizing conditions, we noted that the supernatant turns from a clear, colorless liquid to an optically transparent blue solution and subsequently to a yellow color after weeks. This blue color is characteristic of a family of compounds, known as Molybdenum Blue, that contain predominantly anionic Mo(VI) oxides. We have been unable to crystallographically characterize the precipitate due to its extreme hydroscopic character and because the large hydrating sphere induces phase changes over 344
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Chemistry of Materials the course of measurements. We anticipate that the formation of structural defects and oxidized Mo6+ within the layer and at its edge, as well as the potential adsorption of highly anionic molybdates, would reduce layer−layer attraction and facilitate exfoliation. Furthermore, the absorption of the anionic molybdates to dispersed layers would enhance colloidal stability and account for the net negative charge of the MoS2 in NMP (measured here as −45 mV; reported range in the literature −40 to −50 mV27,28). Finally, the oxidation at reactive edge sites of MoS2 to MoOx−2 anions and subsequent adsorption at the layer edge or at defect sites on the basal plane would account for the increased population of Mo6+ and depletion of sulfur observed in XPS and EDS. This type of oxidation would not affect the optical or structural properties, as the oxidized species are adsorbed to the surface and do not participate in lattice substitutions or orbital mixing. In summary, our analysis indicates that NMP is a good solvent for ultrasonic exfoliation due to the formation of highly reactive hydroperoxides. The primary role of sonication is to facilitate the autoxidation of NMP rather than to generate extreme mechanical forces to drive layer separation. Thus, NMP processing, such as reflux conditions, that favors formation of hydroperoxides should also facilitate exfoliation of MoS2. This idea is supported by Figure 7, which shows MoS2
exfoliation in NMP and DMF (10 mL) in the absence of probe sonication. MoS2 powder (100 mg) and solvent (10 mL) containing H2O (25 mmol) and NMS (3 mmol) were initially heated for 1 h at 100 °C. After cooling, the suspension was centrifuged at 5000 rpm, oxidized solvent was removed by decanting, and fresh anhydrous NMP or DMF was added. Following homogenization in a bath sonicator (ca. 60 s), the suspension was centrifuged at 5000 rpm and the supernatant containing MoS2 flakes was collected and analyzed by UV−vis spectroscopy. Conceptually, autoxidation of the solvent in the presence of the proper substrates will form hydroperoxides during heating, which will oxidize MoS2, leading to exfoliation without aggressive mechanical treatment. Due to the oxidation of Mo4+ and absorption of MoOx−2 clusters, exfoliation should be retained upon replacement of this reaction medium with fresh anhydrous solvent. As a baseline, heating anhydrous NMP in the absence of the necessary substrates (i.e., argon) resulted only in a slight discoloration of the solvent and no exfoliated flakes in the supernatant. In contrast, heating anhydrous NMP in ambient (i.e., O2) produced highly viscous, strongly discolored amber solvent. After removal of this thermal-processed solvent and addition of fresh anhydrous NMP, only a small amount of exfoliated MoS2 (2 μg/mL) was isolated. However, heating NMP in the presence of both substrates (O2 and H2O) resulted in a less viscous yellow solvent and an order of magnitude more exfoliation of MoS2 upon addition of fresh anhydrous NMP (20 μg/mL). Similarly, when anhydrous DMF was heated under an ambient atmosphere, MoS2 exfoliation was ∼4 μg/mL, whereas addition of NMS and H2O to DMF yielded 4 times the concentration (16 μg/mL). We also observed slight exfoliation of MoS2 in anhydrous DMF under an inert atmosphere (∼4 μg/mL), which was similar to that with anhydrous DMF under ambient conditions. Given the structural similarities with NMP, this suggests a similar autoxidative pathway exists in N-alkyl amides, such as DMF. Such hydroperoxide products have been demonstrated in N,N-alkyl amides either through homolytic cleavage or autocatalytic decomposition.56 Finally, the in situ generation of hydroperoxides should occur in the absence of MoS2. Thus, a stepwise preparation of a solution containing hydroperoxides, followed by addition of MoS2, should also produce exfoliated flakes (Figure 7B). Such an activated NMP mixture with hydroperoxides (NMP*) was formed by heating anhydrous NMP (10 mL) with H2O (500 μL, 25 mmol, 1:4 mol ratio H2O/NMP) at 75 °C for 3 h under an ambient atmosphere.36,37 After cooling to room temperature, MoS2 powder was immediately added (150 mg in 5 mL of anhydrous NMP, 10 mg/mL final concentration) while flushing with argon. An initial increase in MoS2 exfoliation that saturates after ca. 15 min was observed, as shown in Figure 7A. Overall, the in situ generation of hydroperoxides followed by solvent removal and addition of fresh anhydrous solvent enables facile formulation of highly concentrated solutions. Additionally, this approach allows for solvent transfer protocols, in which the oxidation occurs in NMP or DMF and exfoliated flakes are resuspended into a more processable solvent, such as ethanol or ACN. These solvents will greatly enhance photophysics investigations, such as photoluminescence, since a reduction in fluorescence quenching is commonly encountered in NMP.
Figure 7. Probeless exfoliation of MoS2. Samples were stirred at 100 °C with the indicated additives (mmol). MoS2 suspensions were obtained after removal of the thermally processed solvent and addition of anhydrous solvent followed by brief homogenization and centrifugation at 5000 rpm. (A) Under the proper oxidative conditions that promote the formation of hydroperoxides, exfoliation can be accomplished without probe sonication. The requirements, discussed in the text, are consistent with conditions that are present during probe sonication. (B) Exfoliation of MoS2 in NMP* (NMP and H2O at 75 °C, 3 h). Thermally heating NMP forms hydroperoxides, allowing for exfoliation of MoS2 at room temperature. 345
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CONCLUSIONS
AUTHOR INFORMATION
Corresponding Author
The experiments presented here challenge the conventional explanation of liquid phase exfoliation based on matching solvent-surface interfacial energies, which results in a low enthalpy of mixing and allows for favorable solvent interactions to disrupt the weak van der Waals forces between TMD layers. The best solvents are identified as those that closely match the interfacial tension, with NMP being a good solvent for MoS2, graphene, and other TMDs. However, this framework does not explain recent literature reports,29,33,57 net surface charge reported for exfoliated flakes,27,28 or findings reported herein. The amount of water introduced into the system has a minimal effect on solubility parameters and thus facial energy matching is insufficient to explain the increased reaction yields. We find that exfoliation yields are better correlated to the formation of highly reactive hydroperoxides in NMP under sonolytic conditions, suggesting that a chemical mechanism underlies NMP’s high activity in exfoliation across TMDs and graphene. Specifically, our findings indicate that the activity of NMP and NMP-like substrates to exfoliation of MoS2 depends on (1) availability and concentration of an appropriate substrate, (2) an oxidizing atmosphere, and (3) H2O. Removal of any one of these components greatly reduces the exfoliation yield, and the inclusion of all three components resulted in the highest yields. Understanding the chemical mechanism behind the activity of NMP also enables a systematic optimization of exfoliation yields via the intentional addition or formation of these hydroperoxides in traditional nonsolvents (e.g., ACN). Introducing reactive species in the absence of probe-tip sonication allows for the oxidation chemistry to occur under relatively mild conditions (e.g., benchtop stirring), and doing so after resuspension in fresh solvent (effectively removing the oxidants and degraded solvent) allowed for probeless exfoliation procedures to be developed. In summary, the aforementioned results indicate that probe sonication is not necessary for efficient exfoliation of MoS2, which may enable new technologies for efficient protocols for TMD exfoliation. Understanding the chemistry that is accompanied by exfoliation allows for shorter reaction times as well as facile and logical scale-up procedures to obtain highconcentration dispersions as well as switching processing solvents to something more conducive for applications (e.g., ethanol, ACN). The removal of probe sonication also results in a less energy intensive process and higher throughput and scaleup. Finally, understanding the surface chemical species and their origins will guide future investigation of reversion chemistries to reduce defects and remove surface adsorbed species to enhance optical and electronic performance of liquid phase exfoliated MoS2 in devices.
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*E-mail:
[email protected]. Phone: 937-785-9209. Fax: 937-656-6327. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS The authors acknowledge the support of this research by the Air Force Office of Scientific Research (AFOSR) and the Air Force Research Laboratory’s Materials and Manufacturing Directorate and express appreciation to Elizabeth Moore and Timothy Prusnik for help with Raman experiments.
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REFERENCES
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ASSOCIATED CONTENT
S Supporting Information *
The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.chemmater.5b04224. Additional details on the determination of molar absorptivity, characterization (UV−vis spectra, FTIR spectra, Raman spectra, XPS spectra, and 1H and 13C NMR), and microscopy (PDF) 346
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