Surface Tension Components Ratio: An Efficient Parameter for Direct

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Surface Tension Components Ratio: An Efficient Parameter for Direct Liquid Phase Exfoliation Man Wang, Xiaowei Xu, Yuancai Ge, Pei Dong, Robert Baines, Pulickel M Ajayan, Mingxin Ye, and Jianfeng Shen ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.6b16578 • Publication Date (Web): 27 Feb 2017 Downloaded from http://pubs.acs.org on March 5, 2017

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ACS Applied Materials & Interfaces

Surface Tension Components Ratio: An Efficient Parameter for Direct Liquid Phase Exfoliation

Man Wanga, Xiaowei Xua, Yuancai Gea, Pei Dongb, Robert Bainesb, Pulickel M. Ajayanb, Mingxin Yea*, and Jianfeng Shena,b*

a

Institute of Special Materials and Technology, Fudan University, Shanghai 200433,

China, [email protected], [email protected]

b

Department of Materials Science and NanoEngineering, Rice University, 6100 Main

Street, Houston, TX 77005, USA

KEYWORDS: two dimensional materials; liquid phase exfoliation; polar and dispersive components; surface tension components ratio; cosolvent

1

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ABSTRACT: Direct liquid phase exfoliation (LPE) is generally regarded as an effective and efficient methodology for preparing single- to few-layered nanosheets on a large scale. Based on previous finding that the polar and dispersive components of surface tension can be used as critical parameter for screening suitable solvents for LPE, in this study, we have conducted in-depth research on direct LPE of two dimensional (2D) materials by the extensive LPE of a series of 2D materials and the thorough comparison of their surfaces properties and LPE efficiencies. We have rationally developed the surface tension component matching (STCM) theory, and in nature its key point lies in the close ratio of polar to dispersive components (P/D) between the solvents and the aimed 2D materials. To this end, surface tension components ratio is demonstrated to be an effective parameter for screening LPE solvents. Besides to the optimization of the LPE process for these 2D materials, this work has further greatly enlarged the comprehensive library for the solvents and 2D materials matching pairs based on the improved STCM theory.

Introduction Two-dimensional (2D) materials, such as graphene, phosphorene, hexagonal boron nitride (h-BN), and metal chalcogenides (MoS2, WS2, MoSe2, WSe2), have attracted the world’s attention over the past few years. High yield preparation of 2D materials has been the prerequisite for their applications in sensors, catalysis, spintronics, optoelectronics, field effect transistors, energy storage devices, and many other applications.1-6

Generally, single- and few-layered 2D nanomaterials can be produced through two methods, i.e., the bottom-up synthesis such as epitaxial growth and chemical vapor 2


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deposition (CVD), and the top-down exfoliation from bulk 2D materials by the means of micromechanical cleavage, chemical and electrochemical exfoliation, and liquid phase exfoliation (LPE).7-9 Among these methods, CVD creates high quality 2D materials but requires harsh growth conditions and additional transfer processes, both of which may introduce residues and defects into the microstructure, resulting in performance deterioration.9-10 Moreover, mechanical cleavage can only be applied in fundamental studies due to its extremely low yield.7 And, although chemical and electrochemical exfoliation produce 2D materials with high yield of monolayers, the resulting materials fail to retain their structure integrity and bring about a host of safety concerns.11-12 The negative aspects of these methods of 2D materials preparation have compelled researchers to shift their focus to direct LPE, a technique initiated by J. N. Coleman et al. to exfoliate powered graphite.13

Actually, LPE can be enhanced by the addition of surfactants, polymers, pyrene derivatives, inorganic salts, and intercalants,14-18 while many such chemical solvents are, to a fair degree, expensive and detrimental to human health or sample quality.7-9 To circumvent cost and safety drawbacks, direct LPE in solvents without the introduction of extra chemicals appeals to many scholars. By direct LPE, 2D nanosheets have been successfully exfoliated in not only solvents with weak volatility, such as N-methyl-2-pyrrolidone, N,N-dimethylformamide, ortho-dichlorobenzene, and pentafluorobenzonitrile,13, 19-20 but also solvents with low boiling points such as chloroform, propanol, and acetonitrile.21-23 In fact, direct LPE boasts a host of merits, such as general facility, environmental friendliness, and scale-up amenability.7, 24-26

Meanwhile, researchers are screening promising solvents and probing into the

mechanisms of LPE. Major efforts have been focused on specific parameters like total 3



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surface tension, Hilderbrand solubility parameters, and Hansen parameters.13,

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Nevertheless, these parameters are still not universal enough in the prediction of good solvents or the interpretation of LPE process.29 In fact, an ideal solvent for LPE would induce reduced potential energy between adjacent layers of 2D materials in order to overcome the van der Waals forces. Simultaneously, interactions between liquid media and layered nanosheets should balance the inter-sheet attractions and stabilize the dispersed nanosheets. Unfortunately, whole mechanism of LPE is currently still unclear and little is known about the details of the interactions between solvents and 2D materials.

Similar to surface energy, surface tension is determined by molecular interactions, which can be divided into polar and dispersive components.30-31 Recently, on the basis of the LPE of graphite, MoS2, WS2, MoSe2, SnS2, TaS2, h-BN, and Bi2Se3 in a series of solvents, we have proposed that both polar and dispersive components exert significant effects on the exfoliation and stabilization of 2D materials and the ratio of polar to dispersive components of surface tension (P/D) can be used as a critical parameter for screening efficient solvents for LPE.32-33 While the detailed process and complete theory are unveiled yet, in this study, on the foundation of further study of other novel layered compounds (WSe2, In2Se3, Sb2S3, In2S3, Bi2Te3, and Sb2Te3) in a series of solvents with varied P/D values, we delineate the process of LPE after comparing, constructing and summarizing the surface property library of 2D materials and cosolvents. To this end, the surface tension component matching (STCM) theory, characteristic of the well matched ratio of polar to dispersive components in surface tension of the solvents and aimed 2D materials, has been greatly developed. Besides, this work further enlarges the comprehensive library for the solvents and 2D materials 4


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greatly.

Experimental Section Materials Preparation Commercially available WSe2 (99.8%, product T119286), In2Se3 (product I119265), In2S3 (99.99%, product I119269), Bi2Te3 (99.99%, product B119271), and Sb2Te3 (99.96%, A119270) were purchased from Aladdin Industrial Corporation, while Sb2S3 (99.995%, product 229466) was supplied by Sigma-Aldrich. Isopropanol (IPA), acetone, and tetrahydrofuran (THF) were bought from Sinopharm Chemical Reagent Co., Ltd. These chemical agents were used without further purification.

LPE process 12 mg bulk 2D materials (WSe2, In2Se3, Sb2S3, In2S3, Bi2Te3, or Sb2Te) were added to a series of 4 mL IPA/water cosolvents with IPA volume fractions of 0, 10%, 20%, 30%, 40%, 50%, 60%, 70%, 80%, 90%, and 100%, respectively (6×11=66 samples in total). The mixtures were batch sonicated in a SB-5200DTD sonicator at the power of 200 W and the frequency of 40 kHz for 4 h, with circulating cooling water to keep the ambient temperature during sonication. Next, the dispersions were centrifuged by a TGL-20B at 4000 rpm for 10 min to remove the remaining un-exfoliated materials. After that, the supernatants were carefully collected for further characterization.

To demonstrate the universal validity of STCM theory, we have also exfoliated Sb2S3 in a series acetone/water and THF/water cosolvents with various surface tension component ratios. And the experiment procedure goes in the same way as that of IPA/water cosolvent system. 5



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Characterization The contact angle data between bulk 2D materials and IPA/water cosolvents were measured with a surface tensiometry DCAT 21. Ultraviolet-visible (UV-vis) absorption spectra of exfoliated 2D material dispersions were obtained through a SHIMADZU UV-3600 with a quartz cuvette (path length 1 cm) in the wavelength range of 200 nm to 800 nm. Atomic Force Microscopy (AFM) results were acquired from a Bruker Multimode 8 in ScanAsyst mode. Before AFM characterization, decuple diluted solutions were spin-coated onto the silicon wafer surfaces by the spin coater SYSC-100S at 6000 rpm. Transmission electron microscopy (TEM) and energy dispersive spectroscopy (EDS) were performed on a JEOL JEM-2010 with an accelerating voltage of 200 kV by drop-casting 2D material dispersions onto the lacy carbon-coated copper grid. Raman spectra were collected by a Dilor LABRAM-1B multi-channel confocal microspectrometer with 514-nm-laser excitation. 2D material dispersion for Raman was spin-coated on the silicon substrate by the spin coater SYSC-100S at 6000 rpm.

Results and Discussion Characterization of surface tension components Although many methods for the exfoliation of bulk 2D materials, such as thermal exfoliation and mechanical exfoliation method, have been successful up to date, yet LPE methods are still required owing to a lot of the future industrial applications of 2D materials in sectors such as large area coatings or composites. Thus, it is quite intriguing to probe into the nature of LPE since it helps to seek answers to a number of fundamental questions about solution processing routes for 2D materials. In previous reports, some certain pure solvents have been employed in LPE successfully 6


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but the majority of them are noxious, expensive, or difficult to recycle. Halim and Zhou have offered a solution to this problem by alternating the cosolvent strategy, in which two or more common solvents synergize in unique mixtures that are universal and feasible for LPE.28, 34 The cosolvent approach will greatly enlarge the library of solvents for LPE by the alteration of constituent proportion in mixtures thanks to the theoretically infinite combination of constituents in mixed solvents.

To investigate the nature of LPE, we are required to conduct fine exfoliation of some certain layered materials and make comprehensive assessment on as-made 2D nanosheets. Therefore, we should first obtain the surface tension components of both the solvents and the bulk 2D materials.

Water is the most widely used solvent, and IPA has also been extensively applied in fundamental research and engineering manufacture in virtue of its hypotoxicity, low cost, and low boiling-point. Actually, IPA/water cosolvent is a favorable mixture system.35 Thus, in our study we mainly focused and utilized a series of IPA/water cosolvents with various component fractions as the aimed cosolvents for LPE. Surface tension components of solvents can be easily acquired by a surface tensiometry DCAT 21. Our group’s previous work has obtained the accurate data of the whole surface tension, polar and dispersive components, as well as their ratios, (P/D)S, of IPA/water, acetone/water, and THF/water cosolvent systems with IPA, acetone, or THF volume fractions of 0, 5%, 10%, 15%, 20%, 30%, 40%, 50%, 60%, 70%, 80%, 90%, and 100%,32 which would be referred in this study.

However, database of the surface energy of solid is limited, let alone their 7



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corresponding surface tension components.35 In addition, published data of solid surface tension vary greatly due to different measurement and calculation methods.36-37 To avoid discrepancies, we would rather adopt a component ratio in lieu of their absolute values. We measured the contact angle data between bulk 2D materials (WSe2, In2Se3, Sb2S3, In2S3, Bi2Te3, or Sb2Te3) and a series of IPA/water cosolvents with the IPA volume fraction of 5%, 10%, 20%, 30%, 50%, and 70%. At least 5 data of contact angle were recorded for each solid material and each cosolvent. Figure 1 shows both average values (the histogram) and standard deviations (the error bar) of these contact angles. Additionally, the contact angle histograms are filled with specific colors for different IPA/water mixtures. As for different 2D materials and IPA/water mixture systems, their contact angles vary severely, influencing further dispersion and exfoliation process.

Based on above contact angle data of solid materials and a series of cosolvents, we further obtained the surface tension component ratio of bulk 2D materials, (P/D)B, through the linear fitting according to the Owen, Wendt, Rabel, and Kaelble (OWRK) equation33 — that is, 0.477 for WSe2, 0.539 for In2Se3, 0.490 for Sb2S3, 0.408 for In2S3, 0.478 for Bi2Te3, and 0.491 for Sb2Te3 (Figure 2), where the y-axis error bars represent the standard deviation arising from 5 measurements). It is obvious that various compounds possess different P/D values and most of them rank in the range of 0.3 to 0.6, indicating the excellent dispersibility in IPA/water cosolvents with IPA fractions of 20% to 80%.

Characterization of exfoliated 2D materials This section concerns the LPE of bulk 2D materials in IPA/water cosolvents. WSe2, 8


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In2Se3, Sb2S3, In2S3, Bi2Te3, or Sb2Te3 was sonicated and exfoliated in a series of 0, 10%, 20%, 30%, 40%, 50%, 60%, 70%, 80%, 90%, and 100% IPA/water cosolvents. Afterwards, the obtained colloidal suspensions were centrifuged to get rid of large sediments. Figure S1 demonstrates 6×11 dispersions of these 2D materials exfoliated in eleven IPA/water cosolvents. It is obvious that the color of the dispersions varies with IPA volume fraction and the darkest-colored ones are marked by blue rectangles. We then conducted UV-vis absorbance tests to characterize the suspension dispersibility. Figure S2 profiles the typical UV-vis spectra of six dispersions, in which the top right insets are the samples exfoliated in the corresponding optimal cosolvents with the most matched P/D values. We adopt the absorbance divided by cell length at certain wavelengths (A/l) in the UV-vis spectra to represent the concentration of exfoliated dispersions, as listed in Table S1.

Figure 3 portrays the dispersibility of 14 materials in IPA/water cosolvents with IPA fractions from 0 to 100%. The data of h-BN, graphene, WS2, MoS2, Bi2Se3, MoSe2, SnS2, and TaS2 were obtained from previous work32. Here, a total of 14 materials are listed in a rough order of band gap, according to a large body of literatures.38-52 Diverse compounds disperse and exfoliate differently in cosolvents with varied IPA concentration. The (P/D)B and (P/D)S values, as well as the optimal cosolvents for these 14 materials are listed in the Table 1.

From contact angle tests, linear fitting, as well as LPE experiments, we obtained the P/D values of both layered materials, (P/D)B, and their corresponding optimal solvents, (P/D)S. Figure 4a portrays dispersibility of In2Se3 and Sb2S3 in a series of IPA/water mixtures, reflecting the influence of IPA fractions of cosolvents on the LPE. While 9



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those for WSe2, In2S3, Bi2Te3, and Sb2Te3 can be seen in Figure S3. We can see that for various bulk 2D materials, their best exfoliated dispersions (pink areas in Figure 4a and Figure S3) fall in different regions with specific IPA/water ratios. For example, the optimal mixtures for In2Se3 and Sb2S3 are cosolvents with the IPA concentration of 40% and 50%.

Figure 4b and Table 1 depicts (P/D)S and (P/D)B for 14 compounds. The slight disparity in height of (P/D)s and (P/D)B histogram bars of Figure 4b illustrates the close values of (P/D)S and (P/D)B. For instance, the optimal IPA volumetric ratio for the LPE of In2Se3 is 40%, whose component ratio (P/D)S is 0.529. Accordingly, the (P/D)B of layered In2Se3 is 0.539—very close to the former value. While as for Sb2S3, the best IPA/water cosolvent (the pink area in Figure 4a) in LPE is that with 50% IPA, and the (P/D)S equals to 0.482, which is very close to 0.490, the (P/D)B for bulk Sb2S3. Such data have verified the solvent selection theory of LPE: the close (P/D)S and (P/D)B values are further indicative of the extent of LPE.

After LPE, AFM was applied to represent the micro morphology of 2D nanosheets in both the optimal and non-optimal cases. We carried out detailed analysis on AFM images as to compile statistics of flake thickness and length as well as to investigate the LPE degree of each 2D materials. Over one hundred nanosheets of each sample were randomly selected and carefully analyzed while observing lateral thickness and length.

Figure 5a-f displays typical AFM topographies and size distribution of nanosheets in their optimal dispersions. From the thickness (in blue) and length (in red) histograms 10


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bars, we find that the majority of laterals run a gamut from 200 to 500 nanometers in length and one to eight nanometers in thickness. And the top left insets are the corresponding AFM images of relatively thin individual nanosheets, specifically WSe2 (ca. 1.6 nm), In2Se3 (ca. 2.1 nm), Sb2S3 (ca. 1.7 nm), In2S3 (ca. 2.8 nm), Bi2Te3 (ca. 4.1 nm), and Sb2Te3 (ca. 4.4 nm). Note that the reported thickness of monolayers is generally 0.6~1 nm.8 Even though there are only about 5% monolayers in all of these sample, more than 40% of these flakes have fewer than 5 layers, and 95% fewer than 10 layers.

The AFM results of Sb2S3 in non-optimal cosolvents (90% IPA) are presented in Figure 5g. From the perspective of length, the samples are a little larger and much thicker than those in optimal cases. Specifically, the ratio of samples thinner than 10 nm in thickness is only 6%, far lower than that in well matched cases (90%). And the typical AFM image illustrates a Sb2S3 sheet with the thickness of 41.6 nm. It turns out that badly matched solvents will not benefit to the LPE process much and result in poor quality products. The above AFM results have confirmed that well matched P/D values of layered materials and solvents contribute to better LPE effect.

We have also investigated the impact of the sonication time on LPE effect. The UV-vis spectra and the Abs (a.u.) data at the characteristic peak 240 nm of Sb2S3 are presented in Figure S4. Although the longer the sonication time lasts, the lager the concentration is, yet the concentration increases more and more slower. We have also studied the size distribution of nanosheets. For Sb2S3 exfoliated in optimal cosolvents and sonicated for 6 hours (Figure 5h), the flake thickness is a little thinner while the sheet length is much smaller, which is derived from severe sheer in sonication.53 That 11



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is to say, to sonicate for too long time is not necessarily benefit to LPE results. In contrast, for Sb2S3 exfoliated in optimal cosolvents sonicated for 2 hours (Figure 5i), the flakes intend to be much thicker and larger and it should be applied more sonication to obtain better exfoliated nanosheets. Therefore, a moderate sonication time is a must for perfect LPE. In this study, the best sonication time is 4 hours. To further investigate the method to control layer number of nanosheets, we conducted further centrifugation treatment for LPE of Sb2S3 in 50/50 IPA/water cosolvent. Figure S5 shows the distribution of Sb2S3 flake thickness and length. It is found that by precisely controlling centrifugal rate of LPE in optimal solvent, we are able to obtain monodispersed nanosheets.

It is viable to investigate the state of these six optimal dispersions with TEM by drop-casting them onto the carbon-coated holey copper grid. TEM and EDS images in Figure 6 and Figure S6 exhibit the composition and morphology of the as-fabricated 2D nanomaterials. In the low magnification photos (top left insets in Figure 6), the lateral size of the nanosheets is 200 to 500 nanometers. Both the selected area electron diffraction (SAED) images in the top right and the high-resolution transmission electron microscopy (HRTEM) graphs illustrate the maintenance of hexagonal symmetry in these six 2D materials. The consistency between SAED and HRTEM imaging affirms that our exfoliated materials are of high structural quality. Additionally, data of lattice spacing in HRTEM are carefully measured and noted. They are as follows: 0.28 nm for WSe2 (100), 0.36 nm for In2Se3 (110), 0.33 nm for Sb2S3 (110), 0.30 nm for In2S3 (211), 0.22 nm for Bi2Te3 (110), and 0.38 nm for Sb2Te3 (101). The existence of these 2D materials can be further validated by Raman results (Figure S7). 12


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Furthermore, we have also exfoliated Sb2S3 in a series acetone/water and THF/water cosolvents with various surface tension component ratios. Figure S8 demonstrates the photograph of Sb2S3 dispersions exfoliated in acetone/water and THF/water cosolvents; while Figure S9 portrays the dispersibility of Sb2S3 nanosheets in these mixtures. It is obvious that the Sb2S3 dispersibility changes with the cosolvent fractions. From above experiment, we have learnt that (P/D)B of bulk Sb2S3 is 0.490. For acetone/water mixture, the optimal solvent for Sb2S3 is 50/50 acetone/water, and the (P/D)S equals to 0.490, the exact value of (P/D)B for bulk Sb2S3. While the best THF/water solvent for Sb2S3 is that with 40% THF, and the (P/D)S equals 0.506, very close to 0.490. It is shown that the close (P/D)S and (P/D)B values are further indicative of the extent of LPE. From above experiment and discussion, we can safely draw the conclusion that the surface tension components ratio is a universal parameter to screen LPE solvents.

The study of surface chemistry and above experiment confirm the significance of surface tension components in immersion (adsorption and spreading) and insertion (intercalation and adhesion)

54-57

, and thus the blueprint of LPE can be drawn as

Figure 7 (to see more details in Supporting Information). Most fundamentally, it indicates that solution thermodynamics and surface tension components can be used as a framework to describe the LPE of 2D materials. Only those solvents which have the most matched surface tension components can complete the whole LPE process successfully. Such a study would be useful for the further detailed insight into the mechanism of LPE, which would allow both the optimization of the LPE process and the discovery of new solvents.

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Conclusions In this study, we have conduced LPE of 2D materials (WSe2, In2Se3, Sb2S3, In2S3, Bi2Te3, and Sb2Te3) in a series cosolvents with various polar to dispersive component ratio of surface tension. Based on multifarious characterization and comprehensive comparison of 14 types of 2D materials and series of cosolvents, the STCM theory for direct LPE was greatly developed. It can be concluded that the LPE efficiency can be greatly enhanced by matching the ratio of surface tension components of cosolvents to that of the aimed 2D material. To this end, this study further enlarges the library of matched 2D materials and solvents, as well as providing a more profound understanding of the mechanism of LPE.

ASSOCIATED CONTENT Supporting Information More characterization of the 2D materials after proper LPE process. More discussion about surface chemistry and LPE process. This material is available free of charge via the Internet at http://pubs.acs.org. Corresponding Author *[email protected], [email protected] Notes The authors declare no competing financial interest.

Acknowledgements This work was financially supported by National Natural Science Foundation of China (51202034).

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References: 1. He, Q.; Zeng, Z.; Yin, Z.; Li, H.; Wu, S.; Huang, X.; Zhang, H. Fabrication of Flexible MoS2 Thin-Film Transistor Arrays for Practical Gas-Sensing Applications. Small 2012, 8, 2994-2999. 2. Alsaif, M. M. Y. A.; Chrimes, A. F.; Daeneke, T.; Balendhran, S.; Bellisario, D. O.; Son, Y.; Field, M. R.; Zhang, W.; Nili, H.; Nguyen, E. P.; Latham, K.; Van Embden, J.; Strano, M. S.; Ou, J. Z.; Kalantar-Zadeh, K. High-Performance Field Effect Transistors Using Electronic Inks of 2D Molybdenum Oxide Nanoflakes. Adv. Funct. Mater. 2016, 26, 91-100. 3. Alsaif, M. M. Y. A.; Field, M. R.; Daeneke, T.; Chrimes, A. F.; Zhang, W.; Carey, B. J.; Berean, K. J.; Walia, S.; Van Embden, J.; Zhang, B.; Latham, K.; Kalantar-Zadeh, K.;

Ou,

J.

Z.

Exfoliation

Solvent

Dependent

Plasmon

Resonances

in

Two-Dimensional Sub-stoichiometric Molybdenum Oxide Nanoflakes. ACS Appl. Mater. Interfaces 2016, 8, 3482-3493. 4. Cao, L.; Yang, S.; Gao, W.; Liu, Z.; Gong, Y.; Ma, L.; Shi, G.; Lei, S.; Zhang, Y.; Zhang, S.; Vajtai, R.; Ajayan, P. M. Direct Laser-Patterned Micro-Supercapacitors from Paintable MoS2 Films. Small 2013, 9, 2905-2910. 5. Gu, X.; Cui, W.; Li, H.; Wu, Z.; Zeng, Z.; Lee, S.-T.; Zhang, H.; Sun, B. A Solution-Processed Hole Extraction Layer Made from Ultrathin MoS2 nanosheets for Efficient Organic Solar Cells. Adv. Energy Mater. 2013, 3, 1262-1268. 6. Kaur, H.; Yadav, S.; Srivastava, A. K.; Singh, N.; Schneider, J. J.; Sinha, O. P.; Agrawal, V. V.; Srivastava, R. Large Area Fabrication of Semiconducting Phosphorene by Langmuir-Blodgett Assembly. Sci. Rep. 2016, 6, 34095. 7. Niu, L.; Coleman, J. N.; Zhang, H.; Shin, H.; Chhowalla, M.; Zheng, Z. Production of Two-Dimensional Nanomaterials Via Liquid-Based Direct Exfoliation. Small 2016, 15



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Page 16 of 30

12, 272-293. 8. Niu, L.; Li, K.; Zhen, H.; Chui, Y. S.; Zhang, W.; Yan, F.; Zheng, Z. Salt-Assisted High-Throughput

Synthesis

of

Single-

and

Few-Layer

Transition

Metal

Dichalcogenides and Their Application in Organic Solar Cells. Small 2014, 10, 4651-4657. 9. Mansukhani, N. D.; Guiney, L. M.; Kim, P. J.; Zhao, Y.; Alducin, D.; Ponce, A.; Larios, E.; Yacaman, M. J.; Hersam, M. C. High-Concentration Aqueous Dispersions of Nanoscale 2D Materials Using Nonionic, Biocompatible Block Copolymers. Small 2016, 12, 294-300. 10. Najmaei, S.; Liu, Z.; Zhou, W.; Zou, X.; Shi, G.; Lei, S.; Yakobson, B. I.; Idrobo, J. C.; Ajayan, P. M.; Lou, J. Vapour Phase Growth and Grain Boundary Structure of Molybdenum Disulphide Atomic Layers. Nat. Mater. 2013, 12, 754-759. 11. Becerril, H. A.; Mao, J.; Liu, Z.; Stoltenberg, R. M.; Bao, Z.; Chen, Y. Evaluation of Solution-Processed Reduced Graphene Oxide Films as Transparent Conductors. ACS Nano 2008, 2, 463-470. 12. Stankovich, S.; Dikin, D. A.; Piner, R. D.; Kohlhaas, K. A.; Kleinhammes, A.; Jia, Y.; Wu, Y.; Nguyen, S. T.; Ruoff, R. S. Synthesis of Graphene-Based Nanosheets Via Chemical Reduction of Exfoliated Graphite Oxide. Carbon 2007, 45, 1558-1565. 13. Hernandez, Y.; Nicolosi, V.; Lotya, M.; Blighe, F. M.; Sun, Z.; De, S.; Mcgovern, I. T.; Holland, B.; Byrne, M.; Gun'ko, Y. K.; Boland, J. J.; Niraj, P.; Duesberg, G.; Krishnamurthy, S.; Goodhue, R.; Hutchison, J.; Scardaci, V.; Ferrari, A. C.; Coleman, J. N. High-Yield Production of Graphene by Liquid-Phase Exfoliation of Graphite. Nat. Nanotechnol. 2008, 3, 563-568. 14. Lotya, M.; Hernandez, Y.; King, P. J.; Smith, R. J.; Nicolosi, V.; Karlsson, L. S.; Blighe, F. M.; De, S.; Wang, Z.; Mcgovern, I. T. Liquid Phase Production of Graphene 16


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by Exfoliation of Graphite in Surfactant/Water Solutions. J. Am. Chem. Soc. 2009, 13, 3611-3620. 15. Smith, R. J.; King, P. J.; Lotya, M.; Wirtz, C.; Khan, U.; De, S.; O'neill, A.; Duesberg, G. S.; Grunlan, J. C.; Moriarty, G.; Chen, J.; Wang, J.; Minett, A. I.; Nicolosi, V.; Coleman, J. N. Large-Scale Exfoliation of Inorganic Layered Compounds in Aqueous Surfactant Solutions. Adv. Mater. 2011, 2, 3944-3948. 16. Dong, X.; Shi, Y.; Zhao, Y.; Chen, D.; Ye, J.; Yao, Y.; Gao, F.; Ni, Z.; Yu, T.; Shen, Z.; Huang, Y.; Chen, P.; Li, L. J. Symmetry Breaking of Graphene Monolayers by Molecular Decoration. Phys. Rev. Lett. 2009, 102, 135501. 17. Niu, L.; Li, M.; Tao, X.; Xie, Z.; Zhou, X.; Raju, A. P.; Young, R. J.; Zheng, Z. Salt-Assisted Direct Exfoliation of Graphite into High-Quality, Large-Size, Few-Layer Graphene Sheets. Nanoscale 2013, , 7202-7208. 18. Kovtyukhova, N. I.; Wang, Y.; Lv, R.; Terrones, M.; Crespi, V. H.; Mallouk, T. E. Reversible Intercalation of Hexagonal Boron Nitride with Bronsted Acids. J. Am. Chem. Soc. 2013, 135, 8372-8381. 19. Hamilton, C. E.; Lomeda, J. R.; Sun, Z.; Tour, J. M.; Barron, A. R. High-Yield Organic Dispersions of Unfunctionalized Graphene. Nano Lett. 2009, 9, 3460-3462. 20. Bourlinos, A. B.; Georgakilas, V.; Zboril, R.; Steriotis, T. A.; Stubos, A. K. Liquid-Phase Exfoliation of Graphite Towards Solubilized Graphenes. Small 2009, 5, 1841-1845. 21. O’neill, A.; Khan, U.; Nirmalraj, P. N.; Boland, J.; Coleman, J. N. Graphene Dispersion and Exfoliation in Low Boiling Point Solvents. J. Phys. Chem. C 2011, 115, 5422-5428. 22. Choi, E. Y.; Choi, W. S.; Lee, Y. B.; Noh, Y. Y. Production of Graphene by Exfoliation of Graphite in a Volatile Organic Solvent. Nanotechnology 2011, 22, 17



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365601. 23. Qian, W.; Hao, R.; Hou, Y.; Tian, Y.; Shen, C.; Gao, H.; Liang, X. Solvothermal-Assisted Exfoliation Process to Produce Graphene with High Yield and High Quality. Nano Res. 2009, 2, 706-712. 24. Hanlon, D.; Backes, C.; Higgins, T. M.; Hughes, M.; O’neill, A.; King, P.; Mcevoy, N.; Duesberg, G. S.; Mendoza Sanchez, B.; Pettersson, H.; Nicolosi, V.; Coleman, J. N. Production of Molybdenum Trioxide Nanosheets by Liquid Exfoliation and Their Application in High-Performance Supercapacitors. Chem. Mater. 2014, 26, 1751-1763. 25. Hai, X.; Chang, K.; Pang, H.; Li, M.; Li, P.; Liu, H.; Shi, L.; Ye, J. Engineering the Edges of MoS2 (WS2) Crystals for Direct Exfoliation into Monolayers in Polar Micromolecular Solvents. J. Am. Chem. Soc. 2016, 138, 14962-14969. 26. Manna, K.; Huang, H.-N.; Li, W.-T.; Ho, Y.-H.; Chiang, W.-H. Toward Understanding the Efficient Exfoliation of Layered Materials by Water-Assisted Cosolvent Liquid-Phase Exfoliation. Chem. Mater. 2016, 28, 7586-7593. 27. Hernandez, Y.; Lotya, M.; Rickard, D.; Bergin, S. D.; Coleman, J. N. Measurement of Multicomponent Solubility Parameters for Graphene Facilitates Solvent Discovery. Langmuir 2010, 26, 3208-3213. 28. Zhou, K. G.; Mao, N. N.; Wang, H. X.; Peng, Y.; Zhang, H. L. A Mixed-Solvent Strategy for Efficient Exfoliation of Inorganic Graphene Analogues. Angew. Chem., Int. Ed. 2011, 50, 10839-10842. 29. Cunningham, G.; Lotya, M.; Cucinotta, C. S.; Sanvito, S.; Bergin, S. D.; Menzel, R.; Shaffer, M. S. P.; Coleman, J. N. Solvent Exfoliation of Transition Metal Dichalcogenides: Dispersibility of Exfoliated Nanosheets Varies Only Weakly between Compounds. Acs Nano 2012, 6, 3468-3480. 18


Page 18 of 30

Page 19 of 30

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30. Owens, D. K. Some Thermodynamic Aspects of Polymer Adhesion. J. Appl. Polym. Sci. 1970, 14, 1725-1730. 31. Jia, L.; Shi, B. A New Equation between Surface Tensions and Solubility Parameters without Molar Volume Parameters Simultaneously Fitting Polymers and Solvents. J. Macromol. Sci., Part B: Phys. 2011, 50, 1042-1046. 32. Shen, J.; Wu, J.; Wang, M.; Dong, P.; Xu, J.; Li, X.; Zhang, X.; Yuan, J.; Wang, X.; Ye, M.; Vajtai, R.; Lou, J.; Ajayan, P. M. Surface Tension Components Based Selection of Cosolvents for Efficient Liquid Phase Exfoliation of 2D Materials. Small 2016, 12, 2741-2749. 33. Shen, J.; He, Y.; Wu, J.; Gao, C.; Keyshar, K.; Zhang, X.; Yang, Y.; Ye, M.; Vajtai, R.; Lou, J.; Ajayan, P. M. Liquid Phase Exfoliation of Two-Dimensional Materials by Directly Probing and Matching Surface Tension Components. Nano Lett. 2015, 15, 5449-5454. 34. Halim, U.; Zheng, C. R.; Chen, Y.; Lin, Z.; Jiang, S.; Cheng, R.; Huang, Y.; Duan, X. A Rational Design of Cosolvent Exfoliation of Layered Materials by Directly Probing Liquid-Solid Interaction. Nat. Commun. 2013, 4, 2213. 35. Girifalco, L. A.; Good, R. J. A Theory for the Estimation of Surface and Interfacial Energies. I. Derivation and Application to Interfacial Tension. J. Phys. Chem. 1957, 61, 904-909. 36. Wang, S.; Zhang, Y.; Abidi, N.; Cabrales, L. Wettability and Surface Free Energy of Graphene Films. Langmuir 2009, 25, 11078-11081. 37. Rafiee, J.; Mi, X.; Gullapalli, H.; Thomas, A. V.; Yavari, F.; Shi, Y.; Ajayan, P. M.; Koratkar, N. A. Wetting Transparency of Graphene. Nat. Mater. 2012, 11, 217-222. 38. Watanabe, K.; Taniguchi, T.; Kanda, H. Direct-Bandgap Properties and Evidence for Ultraviolet Lasing of Hexagonal Boron Nitride Single Crystal. Nat. Mater. 2004, 3, 19



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

404-409. 39. Ji, Y.; Ou, Y.; Yu, Z.; Yan, Y.; Wang, D.; Yan, C.; Liu, L.; Zhang, Y.; Zhao, Y. Effect of Film Thickness on Physical Properties of Rf Sputtered In2S3 Layers. Surf. Coat. Technol. 2015, 276, 587-594. 40. Patel, M. M.; Soni, P. H.; Desai, C. F. Growth and Characterization of Bi2–XSbXTe3 (X=0-0.2) Single Crystals. J. Cryst. Growth 2015, 432, 33-36. 41. Yang, B.; Xue, D. J.; Leng, M.; Zhong, J.; Wang, L.; Song, H.; Zhou, Y.; Tang, J. Hydrazine Solution Processed Sb2S3, Sb2Se3 and Sb2(S1-XSeX)3 Film: Molecular Precursor Identification, Film Fabrication and Band Gap Tuning. Sci. Rep. 2015, 5, 10978. 42. Vijila, J. J.; Mohanraj, K.; Henry, J.; Sivakumar, G. Microwave-Assisted Bi2Se3 Nanoparticles Using Various Organic Solvents. Spectrochim. Acta, Part A 2016, 153, 457-464. 43. Li, H.; Wu, J.; Yin, Z.; Zhang, H. Preparation and Applications of Mechanically Exfoliated Single-Layer and Multilayer MoS2 and WSe2 Nanosheets. Acc. Chem. Res. 2014, 47, 1067-1075. 44. Choi, I. H.; Park, H. J. Pressure Dependence of the Photoluminescence from γ-In2Se3 Thin Films Prepared Using Mocvd with a Single-Source Precursor. J. Korean Phys. Soc. 2014, 64, 1351-1355. 45. Thakur, J.; Saini, H. S.; Singh, M.; Reshak, A. H.; Kashyap, M. K. Quest for Magnetism in Graphene Via Cr- and Mo-Doping: A Dft Approach. Phys. E 2016, 78, 35-40. 46. Beltran-Huarac, J.; Resto, O.; Carpena-Nunez, J.; Jadwisienczak, W. M.; Fonseca, L. F.; Weiner, B. R.; Morell, G. Single-Crystal Gamma-MnS Nanowires Conformally Coated with Carbon. ACS Appl. Mater. Interfaces 2014, 6, 1180-1186. 20


Page 20 of 30

Page 21 of 30

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


47. Zhang, Y. C.; Li, J.; Zhang, M.; Dionysiou, D. D. Size-Tunable Hydrothermal Synthesis of SnS2 Nanocrystals with High Performance in Visible Light-Driven Photocatalytic Reduction of Aqueous Cr(VI). Environ. Sci. Technol. 2011, 45, 9324-9331. 48. Liu, T.; Deng, H.; Cao, H.; Zhou, W.; Zhang, J.; Liu, J.; Yang, P.; Chu, J. Structural, Optical and Electrical Properties of Sb2Te3 Films Prepared by Pulsed Laser Deposition. J. Cryst. Growth 2015, 416, 78-81. 49. Lazar, P.; Martincová, J.; Otyepka, M. Structure, Dynamical Stability, and Electronic Properties of Phases in TaS2 from a High-Level Quantum Mechanical Calculation. Phys. Rev. B 2015, 92, 224104. 50. Lade, S. J.; Uplane, M. D.; Lokhande, C. D. Photoelectrochemical Properties of CdX (X= S, Se, Te) Films Electrodeposited from Aqueous and Non-Aqueous Baths. Mater. Chem. Phys. 2001, 68, 36-41. 51. Di Paola, A.; Addamo, M.; Palmisano, L. Mixed Oxide/Sulfide Systems for Photocatalysis. Res. Chem. Intermed. 2003, 29, 467-475. 52. Dave, M. Optical Analysis for Few TMDC Materials. Bull. Mater. Sci. 2015, 38, 1791-1796. 53. Khan, U.; O'neill, A.; Lotya, M.; De, S.; Coleman, J. N. High-Concentration Solvent Exfoliation of Graphene. Small 2010, 6, 864-871. 54. Bondi, A. The Spreading of Liquid Metals on Solid Surfaces. Surface Chemistry of High-Energy Substances. Chem. Rev. 1953, 52, 417-458. 55. Leger, L.; Joanny, J. F. Liquid Spreading. Rep. Prog. Phys. 1992, 55, 431-486. 56. Fowkes, F. M. Attractive Forces at Interfaces. Ind. Eng. Chem. 1964, 56, 40-52. 57. Comyn, J. Contact Angles and Adhesive Bonding. Int. J. Adhes. Adhes. 1992, 12, 145-149. 21



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Figure 1. Contact angles between bulk WSe2 (a), In2Se3 (b), Sb2S3 (c), In2S3 (d), Bi2Te3 (e), or Sb2Te3 (f), and IPA/water cosolvents with IPA volume fractions of 5%, 10%, 20%, 30%, 50%, and 70%.

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Figure 2. The linear fitting according to the OWRK equation to obtain P/D of bulk WSe2 (a), In2Se3 (b), Sb2S3 (c), In2S3 (d), Bi2Te3 (e), and Sb2Te3 (f). Herein, θ is solid-liquid contact angle, σ s and σ l are the surface tensions of the solid and liquid,

σ sl represents their interfacial surface tension, σ sd and σ sp are the dispersive component and polar component in surface tension of the solid material, and σ ld and

σ lp are those of the liquid solvent.

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Figure 3. Dispersibility of graphene, Bi2Te3, TaS2, Bi2Se3, Sb2Te3, MoSe2, WSe2, MoS2, WS2, In2Se3, Sb2S3, SnS2, In2S3, and h-BN in IPA/water cosolvents with varied volume fraction.

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Figure 4. (a) Dispersibility of In2Se3 and Sb2S3 in a series of IPA/water cosolvents; (b) P/D of both bulk 2D materials (P/D)B and corresponding optimal solvents (P/D)S of graphene, Bi2Te3, TaS2, Bi2Se3, Sb2Te3, MoSe2, WSe2, MoS2, WS2, In2Se3, Sb2S3, SnS2, In2S3, and h-BN.

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Figure 5. AFM images and the counts of flake thicknesses and length of 2D nanosheets dispersed in corresponding optimal solvents sonicated for 4 hours for WSe2 (a), In2Se3 (b), Sb2S3 (c), In2S3 (d), Bi2Te3 (e), and Sb2Te3 (f). And those for Sb2S3 exfoliated in non-optimal cosolvents (90% IPA) sonicated for 4 hour (g). And those for Sb2S3 exfoliated in optimal cosolvents while sonicated for 6 hours (h) and 2 hour (i). Top left insets are typical AFM images of individual nanosheets on silicon wafers.

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Figure 6. TEM of exfoliated WSe2 (a), In2Se3 (b), Sb2S3 (c), In2S3 (d), Bi2Te3 (e), and Sb2Te3 (f) in their optimal dispersions. Top left and top right insets are their corresponding low magnification graphs and SAED images.

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Figure 7. The process of LP: immersion, insertion, exfoliation, and stabilization.

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Table 1. The (P/D)B and (P/D)S values, as well as the optimal cosolvents for graphene, Bi2Te3, TaS2, Bi2Se3, Sb2Te3, MoSe2, WSe2, MoS2, WS2, In2Se3, Sb2S3, SnS2, In2S3, and h-BN 2D Materials Graphene Bi2Te3 TaS2 Bi2Se3 Sb2Te3 MoSe2 WSe2 MoS2 WS2 In2Se3 Sb2S3 SnS2 In2S3 h-BN

(P/D)B 0.471 0.478 1.283 0.916 0.491 0.447 0.477 0.449 0.563 0.539 0.490 0.767 0.408 0.450

optimal volume of IPA (V%) 50 60 5 15 50 80 60 80 50 40 50 20 90 30

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(P/D)S 0.482 0.475 1.240 0.993 0.482 0.450 0.475 0.450 0.482 0.529 0.482 0.719 0.442 0.658


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Surface Tension Components Ratio: An Efficient Parameter for Direct

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