Toward Understanding the Efficient Exfoliation of Layered Materials by

Oct 12, 2016 - Preparation of two-dimensional layered materials (2DLM) nanosheets is critical for both fundamental studies and applications. Here we p...
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Toward Understanding the Efficient Exfoliation of Layered Materials by Water-Assisted Cosolvent Liquid-Phase Exfoliation Kausik Manna, Huin-Ning Huang, Wei-Ting Li, Yao-Huang Ho, and Wei-Hung Chiang* Department of Chemical Engineering, National Taiwan University of Science and Technology, Taipei 10607, Taiwan S Supporting Information *

ABSTRACT: Preparation of two-dimensional layered materials (2DLM) nanosheets is critical for both fundamental studies and applications. Here we present a systematic study to explore the fundamental factors to control the exfoliation of graphite and MoS2 in aqueous N-methylpyrrolidinone (NMP) using water as the cosolvent. Detailed materials characterizations suggested that the cosolvency significantly influences the yield and the stability of exfoliated nanosheets. The dependence of exfoliation on cosolvency was examined by fundamental factors including solid−liquid interfacial parameters, Hansen Solubility Parameter (HSP), and intermolecularinteraction-sensitive physical parameters of the water−NMP mixed solvent system. Fourier transform infrared spectroscopy (FTIR) study revealed that the water−NMP heteroassociated molecular structures, formed with the addition of water to NMP, could play an important role in the liquid exfoliation of layered materials. Our work provides a guide to rational design of a solvent system to improve the yield and stability of the exfoliated materials.

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simply by controlling the ultrasonication27,28 and the centrifugation28 and/or improving the liquid media.15,29,30 While previous reports suggested that long times (up to 400 h) and low speed centrifugation (500−2000 rpm) yield more 2DLM nanosheets,28 the intrinsic properties of the exfoliated nanosheets degrade due to the defects generated during the long time as well as high power sonication.31 Therefore, a properly chosen solvent or solvent system is essential to improve the yield and quality of the exfoliated materials. The suitability of a solvent or a mixture of solvents for LPE basically depends on two major challenges, the exfoliation yield and the stability of exfoliated layers in the given solvent system. According to some previous studies, the yield and stability of exfoliated nanosheets generally depends on three main fundamental factors including solid−liquid interfacial energy, Hansen solubility parameter (HSP), and solvent molecular size.15,17,26,32 A minimum solid−liquid interfacial energy results in the most stable solid−liquid interaction and therefore maximum exfoliation efficiency during LPE.17,33 Recently, Shen et al. reported that surface tension components could be directly probed and matched to predict solvents with effective LPE capability for 2D materials.24 Moreover, HSP and solvent molecular size are also considered to rationalize the stability of the exfoliated nanosheets.15,17,26 Beside this, some other studies also proposed a donor−acceptor relationship (charge transfer

INTRODUCTION Recently nanosheets of two-dimensional layered materials (2DLMs) such as graphene and molybdenum disulfide (MoS2) have introduced a new era in science and engineering because of their exceptional properties for various applications ranging from nanoelectronics, energy storage, and conversion to biomedical applications.1−6 While the successful preparation of 2DLM nanosheets by mechanical exfoliation has spurred intense interest,7,8 it is still challenging to produce those materials with high purity in a bulk quantity. Consequently, the development of a method for the scalable production of 2DLM nanosheets with high quality will lead to important advances in both fundamental studies and innovative applications. As compared to other methods including micromechanical exfoliation,9,10 chemical vapor deposition,11 chemical modification,12 electrochemical exfoliation,13 and ion intercalation,14 liquid-phase exfoliation (LPE) via ultrasonication is one of the most promising ways to produce 2DLM nanosheets with the advantages of low cost, simplicity in operation, and minimal environmental impact.15−18 The yields and stability of the exfoliated nanosheets prepared by LPE are two key factors for the commercialization of 2DLM nanosheets.15,19−21 LPE was first introduced with a successful production of high quality few-layer graphene by sonication in dichlorobenzene.22 Subsequently, a number of studies have explored the possibility to exfoliate layered materials (LM) by various liquid media and techniques for improving exfoliation yields and further understanding of the scientific reasons behind LPE.15,17,23−26 In general, the yields of 2DLMs by LPE can be enhanced © 2016 American Chemical Society

Received: March 24, 2016 Revised: October 11, 2016 Published: October 12, 2016 7586

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Figure 1. Absorbance and exfoliated concentration profiles of graphene (a) and MoS2 (e) as a function of mw in aqueous NMP. All the absorbance values in panels a and e are the average of five individual experiments. The absorption spectra were recorded using six times diluted graphene and MoS2 dispersions, whereas the concentrations of exfoliated dispersions are original. TEM images of few layer graphene (b) and MoS2 (f). HRTEM images of exfoliated graphene (c) and MoS2 (g); inset: fast Fourier transforms of the HRTEM images. Raman spectra of exfoliated graphene (d) and MoS2 (h). The Raman spectra and TEM images of graphene and MoS2 are presented for the samples prepared in 8:2 NMP/water mixed solvent.

via π−π stacking) between solvent and graphene layers27 or the formation of redox-active species by the aerial oxidation of solvent molecules during sonication19 as other factors. Despite those studies, the fundamental reasons to control the LPE are still not clear. In this report, we aim to explore the fundamental factors governing the yield and stability of exfoliated 2DLM nanosheets by LPE. Toward this goal, we have been inspired by previous reports15,17,34 and developed a simple method to synthesize single- and few-layer graphene and MoS2 nanosheets from their bulk materials by a surfactant-free LPE using water as the cosolvent with N-methylpyrrolidinone (NMP). While Nmethylpyrrolidinone (NMP) has been demonstrated as one of the most successful solvents for LPE, the yields of 2DLM nanosheets are still low.15,24,34 Previous work has shown that the stability of reduced graphene oxide dispersion can be significantly enhanced by adding a small amount of water to NMP,35 and the water−NMP cosolvent system has been demonstrated as efficient to produce 2DLM nanosheets by LPE.36 It was found that the cosolvency significantly influences the yield and the stability of exfoliated graphene and MoS2 nanosheets. With a 0.2−0.3 mass fraction of water (mw) in the water−NMP solvent, the final concentration of as-exfoliated graphene and MoS2 nanosheets was enhanced by almost 2.5 times compared with those obtained with pure NMP. Moreover, the single- and few-layer exfoliated graphene and MoS2 nanosheets were found to be highly stable for 18 months in the given solvent system. To reveal the fundamental factors governing the degree of liquid exfoliation and the stability of exfoliated 2DLMs, we concentrate only on the solvent−soluterelated factors by maintaining uniformities in ultrasonication and centrifugation parameters in the present study. We comprehensively examined the dependence of yields of asexfoliated graphene and MoS2 nanosheets on solid−liquid interfacial parameters, HSP, and the intermolecular-interactionsensitive physical parameters including mixing viscosity, excess

molar volume, and excess Gibbs energy of activation of the water−NMP cosolvent systems. We demonstrate that not only solid−solvent interactions but also solvent−solvent interactions directly influence the liquid-phase exfoliation of layered materials. Systematic FTIR study suggested that the water− NMP heteroassociation plays an important role in the exfoliation yield of both the 2DLMs, and for the first time we report that the dependence of the yield on the cosolvent composition is possible to probe by FTIR spectroscopy for water−alkylpyrrolidinone and similar cosolvent systems. Our method should be useful in several aspects. First of all, the use of water as cosolvent with NMP is economical. Second, the water addition efficiently enhances exfoliation yield of the 2DLMs and stabilizes them for a long time. Finally, the number of defects in as-exfoliated graphene nanosheets decreases by the addition of water. Our study also provides a guide to rational design of the cosolvent system to produce single- and few-layer 2DLM nanosheets using LPE.

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RESULTS AND DISCUSSION Nanosheet Preparation and Characterization. Figure 1a,e shows the absorbance as well as concentrations as functions of cosolvent (water) mass fractions (mw) of asexfoliated graphene and MoS2 nanosheets prepared by our LPE method. The photographs and absorption spectra of exfoliated graphene and MoS2 nanosheets are presented as section S1 and Figure S1 in the Supporting Information. Details of the methodology and characterizations are provided in the Experimental Section. The absorbance values are an average of five individual experiments. The exfoliated concentrations of the materials were determined by a filtration and weighing method15,34 using the Lambert−Beer law, A/l = αC, where A/l is the absorbance per cell length. The absorption coefficient, α, was estimated to be 1658 mL mg−1 cm−1 for graphene (at 660 nm34) and 1189 mL mg−1 cm−1 for MoS2 (at 669 nm15). The exfoliated concentrations were found to be increased as the 7587

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Figure 2. (a, b) Photographs of contact angle measurements and (c, d) contact angle (θc) of water−NMP solvents on HOPG (a, c) and natural single crystal MoS2 (b, d). (e) Surface tension (γlg) of water−NMP as function mw at 25 °C. (f) Calculated decrease in solid−liquid interfacial energy (−γlg cos θc) of aqueous NMP on LMs. The contact angle measurements were performed at ambient temperature.

consistent with previous reports.31,34 In addition to the highly intense D band, D′ (1617 cm−1) and D + D′ (2934 cm−1) were also observed, which are associated with the edges or topological defects but not chemical defects or crystal vacancies.31 Two principal modes of Raman shifts A1g and E2g for exfoliated MoS2 nanosheets as well as starting powder materials are represented in Figure 1h. The A1g and E2g modes for bulk MoS2 was found to be at 401 and 375 cm−1, respectively. The Raman shift of A1g mode for exfoliated MoS2 varied in between 404 and 406 cm−1, indicating the presence of single and few layers nanosheets and in agreement with previous reports.15,37 More detailed discussion on Raman spectroscopic characterization of MoS2 are given in Supporting Information (section S3 and Figure S6). Additional X-ray photoelectron spectroscopy (XPS) characterization and further Raman spectroscopic study indicate that the defect densities of the exfoliated graphene nanosheets were increased with sonication time and decreased with water addition in water− NMP mixed solvent system. The results and detailed discussion are provided in the Supporting Information (sections S4 and S5, Figures S7−10). On the basis of the careful materials characterizations, we have developed a simple and efficient method to synthesize graphene and MoS2 nanosheets from their bulk materials by a surfactant-free LPE using water as the cosolvent with NMP. It is also suggested that the as-exfoliated graphene and MoS2 nanosheets uphold their crystalline structures during the exfoliation process with morphology similar to previous studies and retain their typical intrinsic properties.15,34 Solid−Liquid Interaction Parameters. The thermodynamic stability of a solid−liquid interaction is often predicted by the calculation of solid−liquid interfacial energy as given by Young’s equation.38

water was added to the NMP, and the optimal mw for maximum exfoliation was 0.2−0.3 for both the materials. After 6 h sonication followed by centrifugation, this water−NMP composition was able to produce 0.43 (∼8.6% by mass) and 0.5 (∼10% by mass) mg mL−1 of as-exfoliated graphene and MoS2 nanosheets, respectively. Almost 2.5 times the concentrations obtained in the case of pure NMP for both the materials. Significantly the exfoliated nanosheets were highly stable for 18 months at optimal mw (0.2−0.3) (Figure S2, Supporting Information). We further studied the sonication time effect on the exfoliation efficiency and found that sonication time can also influence the increase in exfoliation yield (section S2 and Figure S3, Supporting Information). The result also indicates that that water−NMP mixed solvent at optimal mw (0.2−0.3) gave rise to better yields for every sonication time. Transmission electron microscopy (TEM) was performed to characterize the morphologies of as-exfoliated graphene and MoS2 nanosheets, shown in Figure 1b,c,f,g and Figure S4. TEM and HRTEM images show the lateral size as well as thicknesses of the as-exfoliated graphene and MoS2. The size and layer numbers per nanosheet were determined by the statistical analysis of the TEM images and are presented in Figure S5. Graphene and MoS2 nanosheets predominantly exhibit a lateral size of 500−2000 nm and 50−200 nm, respectively. The layer numbers in the exfoliated nanosheets were estimated by the analysis of edges of the nanosheets15 (Figure S4d,h). Adequate number of monolayer, folded monolayer, and few-layer nanosheets were observed for both the materials. Highresolution TEM (HRTEM) and associated fast Fourier transforms (FFT) images demonstrate that both the asexfoliated graphene (Figure 1c) and MoS2 nanosheets (Figure 1g) show a intrinsic hexagonal structure, indicating no obvious distortion in structure.15,34 Raman spectroscopy was used to study the structures and layer numbers in the exfoliated graphene and MoS2 nanosheets. The exfoliated graphene (Figure 1d) shows three typical peaks assigned as D, G, and 2D bands at 1340, 1577, and 2692 cm−1, respectively, which are

γsl = γsg − γlg cos θc

(1)

where γsl, γsg, γlg, and θc are solid−liquid, solid−gas, liquid−gas interfacial energies, and equilibrium contact angle of liquid on 7588

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Figure 3. Excess thermodynamic quantities: (a) mixing viscosity (Δη), (b) excess Gibbs energy of activation (ΔaG*E), and (c) molar excess volume (VE) of water−NMP solvents. The η, VE, and ΔaG*E data are from previous literature.42,43 FTIR spectroscopy: peak of (d) carbonyl stretching frequency ν(CO), and (e) hydroxyl stretching frequency ν(OH) at different mw (given in the corresponding figures, respectively). (f) Change rate of ν(CO) and ν(OH) shift with respect to mw, d2ν(CO)/(dmw)2 and d2ν(OH)/(dmw)2, respectively, shows mw-dependent behavior.

the solid surface, respectively. In general, a minimum γsl (maximum γlg cos θc) implies thermodynamically more stable solid−liquid interaction. For a given layered material, γlg can be directly obtained from surface tension of the liquid while γsg is constant. Therefore, the variation in γsl with change of solvent system can therefore simply be predicted by determining the γlg and θc. Figure 2 represents the evaluated interfacial parameters including the photographs of contact angle measurements (Figure 2a,b), contact angles (Figure 2c,d), surface tensions (Figure 2e), and the calculated decrease in solid−liquid interfacial energy (−γlg cos θc) of aqueous NMP on LMs (Figure 2f) as functions of mw. Neither graphite nor MoS2 result in minimized −γlg cos θc (maximum γlg cos θc) for maximum exfoliation around 0.2−0.3 mw (Figure 1a, e, and 2f). The −γlg cos θc values of graphite are higher than those of MoS2 for all water-NMP compositions except mw = 0.2 and 0.3. In particular, in the higher water content (mw = 0.4 to 0.9) region, the differences are more prominent. The higher exfoliations of MoS2 compared with those of graphite at mw = 0.4 and 0.5 (Figure 1a,e) could be possibly due to the lower −γlg cos θc at those water−NMP mole ratios (Figure 2f). The −γlg cos θc profiles presented here could also be used to explain the very low exfoliation yields of both materials around 0.6−0.9 mw. Other solid−liquid interaction parameters are also considered to further understand the fundamental factors to control the exfoliation yield. Previous work39 suggests that solid−liquid interfacial work of adhesion (Wsl) can be used to study the solid−liquid interaction in terms of wetting of the solid phase by the liquid phase. In the present study, the Wsl of water− NMP on LMs has also been evaluated to understand the wettability of LMs by water−NMP (see section S6 in Supporting Information). It is found that Wsl values for MoS2 and graphite were very close for mw = 0.2 and 0.3 and that

might explain the similar exfoliations at mw = 0.2 and 0.3 for both the LMs (Figure 1a,e). However, Wsl could not rationalize the water−NMP composition-dependent exfoliation results of the LMs (more discussion in section S6 in Supporting Information). In addition, a recent study has demonstrated the screening of solvents for LPE by using surface tension components based on the OWRK theory.24 However, the evaluation process of surface tension components of mixed solvents is very complicated (see Supporting Information section S7). In short, while solid−liquid interaction parameters including γsl and Wsl could be useful for the selection of liquid media for LPE, they are still not sufficient to rationalize the liquid exfoliation of the LMs.15,17 There should be other governing factors to control the exfoliation efficiency in the present system. Hansen Solubility Parameters (HSPs). HSP is a semiempirical correlation developed to explain solubility behavior. The solubility of a solute in a solvent can be interpreted with the help of HSP by correlating dispersive (δD), polar (δP), and hydrogen bonding (δH) parameters of solvent system and solute.40 The quality of the dissolution process is expressed as HSP distance Ra: Ra = [4(δ D,1 − δ D,2)2 + (δ P,1 − δ P,2)2 + (δ H,1 − δ H,2)2 ]0.5 (2)

where 1 and 2 in subscripts stand for solvent and solute, respectively. A lower Ra value signifies a better dissolution. The HSP parameters of pure solvents can be obtained from the database of a large number of solvents available from various sources40,41 while the HSP for solid solute materials are generally determined experimentally by dispersing those in a series of various solvents.15 Moreover, the HSP calculation would become complicated when a miscible cosolvent is added 7589

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formed due to the maximized association of water−NMP heteroaggregates by H-bonding around 0.2−0.3 mw. To further understand the water−NMP heteroassociation by H-bonding, we have performed the FTIR absorption spectroscopic study of aqueous NMP at various mw. Figure S13 shows the wide scan FTIR spectra of water−NMP cosolvent with varying mw. All the spectra of aqueous NMP exhibit obvious carbonyl stretching ν(CO) ranging from 1640 to 1680 cm−1 (for pure NMP) and hydroxyl stretchings ν(OH) ranging from 3360 (for pure water) to 3480 cm−1. To study the intermolecular interaction in the water−NMP cosolvent system, the peaks of ν(CO) and ν(OH) were normalized and are shown in Figure 3, panels d and e, respectively. We noticed that the ν(CO) of NMP and the ν(OH) of water shift to lower (red shift) and higher (blue shift) frequency regions, respectively, suggesting the heteroassociation of water−NMP due to the intermolecular CO···H−O type H-bonding in aqueous NMP (Figure 3d,e).49,50 Moreover, the blue shift and reduced peak intensities (Figure S13) of ν(OH) further indicate the existence of clathrate hydrophobic hydration, which is consistent with previous reports.49,50 We evaluated the change rate of shift in ν(CO) and ν(OH) with respect to mw, represented as d2ν(CO)/(dmw)2 and d2ν(OH)/(dmw) 2, respectively, which are plotted as functions of mw (Figure 3f). Significantly, it is found that both the profiles show sharp maxima at mw = 0.2−0.3 suggesting that water−NMP heteroassociation predominantly occurred around 0.2−0.3 mw (water/NMP mole ratio 2:1), and one NMP molecule could bind with two water molecules through hydrogen bonding (Figure S14). The (NMP·2H2O)n clathrate type aggregates formed by the water−NMP heteroassociation can be considered as a critical factor to control the exfoliation yield of 2DLMs (see below discussion). Overall, the FTIR result suggests that heteroassociation between water and NMP molecules may influence the LPE of LMs. On the basis of the above study, we propose that the dependence of exfoliation yield on mw could be governed by two factors including the solid−liquid interaction and the water−NMP heteroassociation. First, the favorable solid−liquid interactions between the LM and the solvent system should be provided for improving the exfoliation efficiency by minimizing γsl and maximizing Wsl at the optimal mw (Figure 2f and Figure S11, respectively). Moreover, the existence of clathrate type water−NMP aggregates could stabilize 2DLMs dispersion in two ways by preventing the recombination of exfoliated layers (Figure 3) and by reducing the sedimentation velocity of suspended 2DLM trough viscosity enhancement (Figure 3a and section S9). Figure 4 represents the effect of solvents on the stabilization of exfoliated nanosheets by the formation of (NMP·2H2O)n clathrate aggregates due to water−NMP heteroassociation. The water−NMP heteroassociation directly influences exfoliation by preventing the recombination of exfoliated layers, and the bulky (NMP·2H2O)n aggregates are able to provide intersheet repulsive forces and separate the nanosheets with nonoverlapping Leonard−Jones (L-J) potentials (Figure 4a). On the other hand, at higher mw (>0.5), the excess water molecules undergo self-association rather than heteroassociation with NMP molecules. In this molecular arrangement, NMP molecules hide completely in the water molecular network and cannot interact with hydrophobic LMs51 (Figure 4b). Consequently, the highly water-rich region is unable to produce high liquid exfoliation of LMs due to adverse solid−liquid interaction and the disruption of (NMP·

to a major solvent, especially when water is a cosolvent. Table S3 (Supporting Information) includes data from three different HSP analyses of the behavior of water. The first set of data is for water as single molecule. The second set of data is based on a correlation of the solubility of various solvents in water, where good solvents are soluble to more than 1% in water and poor ones dissolve to a lesser extent. The third set of data is for a correlation of total miscibility of the given solvents with water. In the present study we have all three HSP data of water to examine the effect of HSP on the exfoliation efficiency. Figure S12 (Supporting Information) exhibits that the exfoliation of LMs in water−NMP mixed solvent is not correlated with HSP distance. We suggest that this is the current limitation of HSP to water−based cosolvent systems, and that is the state of art at present. Solvent−Solvent Interaction: Water−NMP Heteroassociation. The stabilization of the exfoliated 2DLM nanosheets is one of the challenging tasks in LPE. Generally, the stability of solute molecules in a solvent results from three main interactions (solvent−solvent, solvent−solute, and solute− solute). We have discussed the solvent−solute interactions in previous sections. In mixed solvent systems the solvent−solvent interactions can make a significant contribution toward solvent−solute interactions and therefore the stability of the solution. Previous studies demonstrated the unusual changes in excess thermodynamic quantities of water-NMP (as well as other water-alkylpyrrolidinone) solvent system depending on the mixed solvent composition.42,43 Excess thermodynamic quantities are the properties of a mixture which characterize the nonideal behavior and give insight into the intermolecular interactions in the mixture42 (see Supporting Information section S8). Figure 3a−c shows the excess thermodynamic quantities of aqueous NMP; herein the mixing viscosity (Δη), molar excess volume (VE), and excess Gibbs energy of activation (ΔaG*E). Δη, VE, and ΔaG*E data are summarized from previous literature42,43 and are also listed in Table S4. The sharp decrease in VE may be considered as a consequence of three facts: (i) strong solvation of NMP in water by hydrogen bonding (H-bonding), with slight destruction of self-associated water structure, or (ii) appropriate fitting of the alkyl moieties into self-associated water cavities caused by interstitial solvation (clathrate formation), or (iii) may be result of both i and ii.44 The sharp increase in ΔaG*E and therefore positive ΔaG*E values for NMP−water mixture indicate specific interactions between water and NMP molecules.45 We found that all the excess thermodynamic quantities (Δη, VE, and ΔaG*E) exhibit their extrema (maximum or minimum) at the same mw (0.3), which is very close to the optimal mw (0.2−0.3) for the exfoliation of LMs (Figure 1a,e). The increase in viscosity of a solvent blend often arises from aggregation.46 Moreover, increased viscosity usually reduces the sedimentation velocity47 of the suspended 2DLMs during centrifugation. The details of the viscosity-dependent sedimentation velocity are given in section S9 (Supporting Information). In general, the relationship between viscosity and sedimentation velocity follows Stokes law, which basically assumes the suspended particles as spherical. In the present study, the concept of Stokes law may not be considered as a dominant factor due to the 2D nanosheet structures of our samples. From the molecular point of view, the origin of the viscosity enhancement due to the water−NMP heteroaggregates may have a more prominent effect on the exfoliation. The results imply that the polymeric species (NMP·2H2O)n48 and/or (NMP·3H2O)n44 are possibly 7590

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molecules at the premicellar region and the hydrophobic NMP molecules possibly interact directly with the hydrophobic LM surface (Figure 5). Our work suggests that the enhancement of the exfoliation of LMs with the addition of water to NMP is mainly driven by the favorable solid−liquid interactions and the stabilization of the exfoliated 2DLMs by aggregates formed by water−NMP heteroassociation.

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CONCLUSION In summary, we report a comprehensive study to rationalize the synthesis of LM nanosheets by liquid exfoliation of LMs using a water−NMP mixed solvent system. The use of water as cosolvent with NMP is economical. An appropriate amount of water not only significantly enhances the exfoliation yields and stabilizes the nanosheets for a long time but also reduces the defect densities of the as-exfoliated nanosheets. Our results suggest that while solid−liquid interfacial parameters are critical for the selection of suitable solvent(s) for LPE of LMs, it is not sufficient to explain the dependence of exfoliation yield on the solvent composition in the case of mixed solvent systems. The systematic FTIR spectroscopic study suggests that the unique molecular aggregates due to NMP−water heteroassociation play an important role during exfoliation. We propose that the water−NMP aggregation is the fundamental origin which influences directly and indirectly (through viscosity enhancement) the stability of 2DLM dispersion. Finally, we conclude that both the solid−solvent interactions and solvent−solvent interactions (NMP−water heteroassociation) are critical factors in the liquid-phase exfoliation of layered materials. The work opens an avenue for a facile and scalable production of fewlayered 2DLM nanosheets by LPE with a subsequent scientific perspective in science and technology.

Figure 4. Schematic presentation of the effect of solvents on the stabilization of exfoliated materials. (a) The stabilization of the exfoliated nanosheets by (NMP·2H2O)n aggregates formed by heteroassociation between water and NMP molecules under favorable solid−liquid interaction, preventing the Leonard−Jones interaction between exfoliated layers. (b) The overlapping of layers by Leonard− Jones interaction at the highly water-rich region due to the disruption of water−NMP aggregated structures and unfavorable solid−liquid interaction.

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2H2O)n aggregates. Previous studies suggested that aqueous surfactant solutions are better exfoliating agents at the premicellar than at the postmicellar region.52 At the premicellar region, the hydrophobic tails of surfactant molecules can interact with hydrophobic LMs and prevent the recombination of exfoliated nanosheets while at the postmicellar region those hydrophobic tails hide inside the micelles, allowing exfoliated nanosheet overlap. In this context, it is rational to assume that the (NMP·2H2O)n aggregates formed act as the surfactant

EXPERIMENTAL SECTION

Materials. Graphite (−325 mesh, 99.995% pure) and MoS2 (−325 mesh, 99% pure) microcrystalline powders were purchased from Alfa Aesar. N-Methylpyrrolidinone (NMP, 99% extra pure) was purchased from Acros Organics. The HOPG (446HP-AB) and single crystal MoS2 (429MM-AB) used for contact angle measurements were purchased from SPI Suppliers (West Chester, PA). Deionized water was used in all experiments.

Figure 5. Heteroassociation between water and NMP molecules at 2:1 mole ratio and formation of (NMP·2H2O)n aggregates. The figure also schematically shows how the aggregates behave like amphiphilic molecules to create intersheet repulsive force. 7591

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Exfoliation. Standard 14 mL centrifuge tubes were used for the exfoliation via sonication and followed by centrifuge processes to avoid material loss during transfer to centrifuge tubes. A 50 mg amount of each material was placed in centrifuge tubes with an initial concentration 5 mg mL−1 for exfoliation. The materials were batch sonicated for 6 h in a bath sonicator (Elma sonic P60H) at a fixed nominal power and frequency 100 W and 37 kHz, respectively. The positions of the each sample tube were changed after every 30 min to achieve uniform power daistribution. The water of the bath sonicator was replaced with normal cold water in every 30 min to control the temperature rise during sonication. The experimental temperature remains in between 27 °C and 37 °C during sonication. Sample dispersions were stored overnight after sonication, and then they were centrifuged at 3000 rpm for 30 min with a Hettich EBA20. The upper 3/4th of the supernatant was collected and kept undisturbed for 24 h for further precipitation if any, and the upper 2/3rd of each supernatant was used for characterization. Every experiment was repeated five times to minimize errors. Materials Characterizations. Ex situ characterizations of the starting materials and as-produced samples were performed by absorbance spectroscopy, TEM, XPS, and micro-Raman spectroscopy. Absorbance spectra of exfoliated dispersions were recorded using a JASCO V676 UV−vis-NIR spectrophotometer in a matched pair of quartz cuvettes of path length 1 cm. The baseline correction was performed using corresponding solvents during every spectral measurement. Cold-field emission Cs-corrected TEM (JEOL ARM200F, Japan) with 200 kV accelerating voltage was used. Carboncoated copper grids (400 mesh) were used in the TEM sample preparation. XPS (VG ESCALAB 250, Thermo Fisher Scientific, UK) was performed with monochromatic Al Kα X-ray radiation (10 kV, 10 mA). The source power was set at 72 W, and pass energies of 200 eV for survey scans and 50 eV for high-resolution scans were used. Raman scattering studies were performed at room temperature with a JASCO 5100 spectrometer (λ = 533 nm). The thin films for XPS and Raman analysis were prepared on a Si wafer and dried in hot air oven at 60 °C. The FTIR spectra of water-NMP as function of mw were recorded on an FTIR-iS10 (Nicolet). All the spectra were recorded at 25 °C. Background corrections were performed before every measurement Contact Angle Measurement. The contact angle of NMP−water mixed solvent at varying mass fractions of water were determined using a video-based optical contact angle meter and contact angle goniometer, model 100SB, Sindatek (Taiwan). The contact angle values were determined by fitting the contact angle photographs with appropriate software. The sampling of the liquid drops on HOPG and single crystal MoS2 surface were performed with a micropipette, and one spot was used once for measurement. All the contact angle measurements were performed at ambient temperature. Each and every contact angle value was the result of an average of five different data points. Surface Tension Measurement. The surface tensions of water− NMP mixed solvents were measured as a function of mw by a ring detachment method using a Du Noüy tensiometer 514-B. The platinum ring and solvent container were cleaned and dried properly before every single measurement. Extra pure (99%) acetone was used to clean the platinum ring. The measurement temperature was maintained at 25 °C throughout the surface tension measurements.

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Article

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the Ministry of Science and Technology (MOST) of Taiwan (MOST grant no. MOST 103-2221-E-011-150-MY2, MOST 104-2923-E-011-001-MY3, and MOST 105-2221-E-011-141). K.M. acknowledges the receipt of a fellowship from MOST of Taiwan. The authors acknowledge the TEM technical assistance of Dr. Cheng-Yu Hsieh and Dr. Shen-Chuan Lo of Industrial Technology Research Institute (ITRI) and contact angle measurement assistance of Prof. Yu-Lin Kuo.

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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.6b01203. Details of the UV−visible, Raman, XPS, FTIR spectroscopic characterizations, work of adhesion, surface tension components, excess thermodynamic quantities, and sedimentation velocity. Sections S1 to S9, Tables S1 to S4, and Figures S1 to S14 (PDF) 7592

DOI: 10.1021/acs.chemmater.6b01203 Chem. Mater. 2016, 28, 7586−7593

Article

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