Aqueous Phase Exfoliation of Two-dimensional Materials Assisted by

Subscriber access provided by READING UNIV

Article

Aqueous Phase Exfoliation of Two-dimensional Materials Assisted by Thermo-responsive Polymeric Ionic Liquid and Their Applications in Stimuli-responsive Hydrogel and Highly Thermally Conductive Film Xiongwei Wang, and Peiyi Wu ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.7b15712 • Publication Date (Web): 02 Jan 2018 Downloaded from http://pubs.acs.org on January 3, 2018

Just Accepted “Just Accepted” manuscripts have been peer-reviewed and accepted for publication. They are posted online prior to technical editing, formatting for publication and author proofing. The American Chemical Society provides “Just Accepted” as a free service to the research community to expedite the dissemination of scientific material as soon as possible after acceptance. “Just Accepted” manuscripts appear in full in PDF format accompanied by an HTML abstract. “Just Accepted” manuscripts have been fully peer reviewed, but should not be considered the official version of record. They are accessible to all readers and citable by the Digital Object Identifier (DOI®). “Just Accepted” is an optional service offered to authors. Therefore, the “Just Accepted” Web site may not include all articles that will be published in the journal. After a manuscript is technically edited and formatted, it will be removed from the “Just Accepted” Web site and published as an ASAP article. Note that technical editing may introduce minor changes to the manuscript text and/or graphics which could affect content, and all legal disclaimers and ethical guidelines that apply to the journal pertain. ACS cannot be held responsible for errors or consequences arising from the use of information contained in these “Just Accepted” manuscripts.

ACS Applied Materials & Interfaces is published by the American Chemical Society. 1155 Sixteenth Street N.W., Washington, DC 20036 Published by American Chemical Society. Copyright © American Chemical Society. However, no copyright claim is made to original U.S. Government works, or works produced by employees of any Commonwealth realm Crown government in the course of their duties.

Page 1 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

ACS Applied Materials & Interfaces

Aqueous Phase Exfoliation of Two-dimensional Materials Assisted by Thermo-responsive Polymeric Ionic Liquid and Their Applications in Stimuli-responsive Hydrogel and Highly Thermally Conductive Film Xiongwei Wang1, Peiyi Wu1,2* 1

State Key Laboratory

of Molecular Engineering of Polymers,

Department of

Macromolecular Science, Fudan University, Shanghai 200433, P. R. China 2

State Key Laboratory for Modification of Chemical Fibers and Polymer Materials, College

of

Chemistry,

Chemical

Engineering

and

Biotechnology,

Center

for

Advanced

Low-Dimension Materials, Donghua University, Shanghai 201620, China

E-mail：[email protected]

Keywords: two-dimensional materials; liquid phase exfoliation; polymeric ionic liquid; smart hydrogel; thermal conductivity

1 ACS Paragon Plus Environment

ACS Applied Materials & Interfaces 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

Abstract: With the increasing attention for various two-dimensional (2D) materials in recent years, developing a universal, facile and eco-friendly method to exfoliate them into singleand few-layered nanosheets is becoming more and more urgent. Herein, we use thermo-responsive polymeric ionic liquid (TRPIL) as a universal polymer surfactant to assist the high-efficiency exfoliation of molybdenum disulfide (MoS2), graphite and hexagonal boron nitride (h-BN) in aqueous medium through consecutive sonication. In this case, the reliable interaction between 2D materials and TRPIL would facilitate the exfoliation and simultaneously achieve the non-covalent functionalization of exfoliated nanosheets. Interestingly, the dispersion stability of exfoliated nanosheet suspensions can be reversibly tuned by the temperature due to the thermo-responsive phase transition behavior of TRPIL. As a proof of potential applications, a temperature and photo dual-responsive TRPIL/MoS2 coloring hydrogel with robust mechanical property and an artificial nacre-like BN nanosheet (BNNS) film with high thermal conductivity were fabricated.


Page 2 of 30

Page 3 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


1. Introduction During the past few years, two-dimensional materials (TDMs) with strong in-plane bonds but weak interlayer interactions, such as graphene, metal chalcogenides (MoS2, WS2 and MoSe2) and hexagonal boron nitride (h-BN), have become one of the most popular research focuses due to their extensive applications in electrochemical/photo catalysis, electronics, material strengthening/functionalization, energy conversion/storage and chemical sensing.1-6 In order to realize the maximal effect, exfoliation of these layered materials into single- or few-layered nanosheets is necessary. To date, great effort has been devoted to developing various strategies for the reliable production of atomically thin 2D nanomaterials, i.e., chemical vapor deposition (CVD), micromechanical cleavage, chemical exfoliation and liquid phase exfoliation (LPE).7-9 In comparison with the other three approaches, LPE, first proposed by Coleman et al. in 2008, appears to be a versatile, facile technique for the scale-up production of defect-free nanosheets.10-12 In the past decade, although various LPE techniques (e.g., ultrasonication, mechanical grinding and vortex fluidic exfoliation) have been developed, direct sonication of TDMs in some appropriate organic solvents which have the similar surface energy to the target materials is still the most widely-used approach.9, 12-13 However, many of these organic solvents with significant exfoliation efficiency have the problems of high boiling point, toxicity and poor dispersion stability for exfoliated nanosheets. Therefore, LPE of TDMs in aqueous medium is naturally expected to have cost-effective and eco-friendly virtues compared with exfoliation in organic solvents.14 Nonetheless, the strongly hydrophobic nature of TDMs is a great challenge for their exfoliation in water. To address this problem, various stabilizers (e.g., surfactants, polymers, 3 ACS Paragon Plus Environment


Page 4 of 30

inorganic salt and strong acid) are incorporated to realize the homogeneous distribution and reliable exfoliation of TDMs in aqueous medium.15-19 Among them, LPE of hydrophobic TDMs assisted by polymeric surfactant in water has attracted the largest interest in recent years since it can not only produce the defect-free, well-dispersed nanosheets but also achieve the noncovalent functionalization of nanosheets by polymer.12, 20 Moreover, the resultant dispersion can be directly used to prepare various multifunctional polymer composites upon removal of the solvent.21-22 During the aqueous exfoliation, the amphiphilic polymer would partially adsorb onto the surface of the exfoliated nanosheets via hydrophobic, π-π, electrostatic or charge transfer interactions to provide steric repulsion, stabilizing the exfoliated nanosheets against aggregation.12, 20 The initial attempt concerning the polymer-assisted LPE of TDMs in water is reported by Bourlinos et al., in which water-soluble polymer PVP was used to directly exfoliate graphite into ultrathin graphene nanosheets.23 Following their work, a number of other water-soluble polymers and bio-macromolecules, such as PVA, F127, PAA, PSS, chitosan, bovine serum albumin (BSA), DNA/RNA nucleotides, lignin, sodium alginate and etc, have been successively utilized to assist the exfoliation and stabilization of 2D nanosheets (e.g., graphene, MoS2, WS2 and BN) in aqueous medium via a facile sonication treatment.14,

24-31

As a result, noncovalent

functionalization of various polymers or natural macromolecules onto the exfoliated nanosheets has been realized which is favorable to their compatibility with polymer matrix or even organisms. Despite of that significant efforts have been made to the polymer-assisted exfoliation of TDMs in water, one polymer that allows a universal exfoliation of various TDMs has not been reported. 4 ACS Paragon Plus Environment

Page 5 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


Unlike organic solvents, ionic liquids (ILs) are organic salts with low melting point that have garnered increasing interest as a green alternative to volatile organic compounds because of their negligible vapor pressure, high thermal stability and non-flammability.32-33 Benefit from these properties, they have also been directly used as a medium to exfoliate layered materials by facile sonication or microwave irradiation method to yield highly concentrated nanosheet dispersion due to the strong cation-mediated interaction between ILs and layered materials.34-36 However, apart from the limitations of large viscosity, high cost obstructed in the ILs-mediated exfoliation, the controversial toxicity of ILs is also an important issue needed to be considered. Because the reputation of ILs as “environmental friendly” chemicals is mainly based on their negligible vapor pressure. Nonetheless, a number of literatures have reported the toxicity of ILs to aquatic organisms due to their good solubility in water.37 Connecting IL monomers through a polymeric backbone to form polymeric ionic liquids (PILs) could significantly decrease their toxicity and simultaneously incorporate the unique properties of ILs into the polymer chains. Among them, thermo-responsive PILs (TRPILs) are one important member of PIL family which is commonly composed of alkylphosphonium or alkylammonium cations with hydrophilic anions (e.g., halogen anion, carboxylate and sulphonate). It is noteworthy that a rational hydrophilic-hydrophobic balance between cation and anion has been considered to be responsible for temperature-sensitive phase transition of PILs in water. To the best of our knowledge, TRPIL as a polymer surfactant to exfoliate the TDMs has never been reported and maybe give rise to some impressive effects.38 Recently, Mandal et al. used two cationic PILs (P[VBTP][Cl] and P[VimBu][Br]) to promote the LPE of MoS2/MoSe2 in aqueous 5 ACS Paragon Plus Environment


medium and found that the obtained MoS2 suspension showed a thermo-responsive feature.39 However, in this system, the selected PILs only allow the aqueous exfoliation of metal chalcogenides and their thermo-responsive property is only triggered in the presence of NaI or NaCl. Additionally, in our previous work, a synthesized thermo-responsive copolymer of poly (N-isopropylacrylamide-co-IL) (PNIL) was also employed as a polymer surfactant to achieve the simultaneous exfoliation and noncovalent functionalization of MoSe2 nanosheets for a temperature and photo dual-responsive nanocomposite hydrogel.40 However, the temperature sensitive function of this hydrogel is mainly supported by the poly (N-isopropylacrylamide) (PNIPAM) moiety rather than the PIL moiety. Therefore, these two works inspire us that by employing a thermo-responsive PIL (TRPIL) with unique structure to assist the LPE of TDMs, it might achieve the reliable exfoliation of different TDMs and meanwhile endow the exfoliated nanosheets with thermo-responsive property for multifunctional application prospects. In this work, a synthesized thermo-responsive poly ([P4,4,4,4][SS]) was selected as the model polymer to assist the liquid phase exfoliation of MoS2, h-BN and graphite in aqueous medium and simultaneously achieve their non-covalent functionalization. In this case, the hydrophobic and cation-mediated interaction between amphiphilic TRPIL and TDMs are favorable to the homogeneous dispersion and effective exfoliation of TDMs in water. Due to the thermo-responsive phase transition behavior of TRPIL, the obtained nanosheet suspensions show reversibly temperature-mediated dispersion stability. As a proof of potential applications, the TRPIL-functionalized MoS2 nanosheets were in-situ incorporated into a TRPIL hydrogel by a one-pot crosslinking polymerization. The resultant TRPIL/MoS2 6 ACS Paragon Plus Environment

Page 6 of 30

Page 7 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


composite hydrogel showed a temperature and photo dual-responsive coloring change feature. Moreover, artificial nacre-like BNNS films with highly thermally conductive performance were also fabricated through a vacuum-filtration-induced layer-by-layer self-assembly of the TRPIL-functionalized BNNSs.

2. Results and discussion

Figure 1. (a) Synthesis route of TRPIL and (b) schematic illustration of TRPIL-assisted exfoliation of h-BN, bulk MoS2 and graphite powders in aqueous medium. The synthesis route of TRPIL and the exfoliation of three representative TDMs are schematically described in Figure 1. Tetrabutylphosphonium bromide ([P4,4,4,4]Br) and sodium 4-vinylbenzene sulfonate (Na[SS]) were firstly mixed to synthesize the 7 ACS Paragon Plus Environment


thermo-responsive IL monomer by the anion exchange reaction at room temperature. The well-defined structure of the synthesized IL monomer is confirmed by the corresponding 1H NMR spectrum (Figure S1). The polymer of TRPIL was further prepared via a simple radical polymerization of IL monomer. After purification, the 1H NMR spectrum of TRPIL shows the complete disappearance of Ha, b, c signals in comparison with that of IL monomer, suggesting the successful synthesis of TRPIL. It is as expected that the TRPIL aqueous solution possesses a reversible lower critical solution temperature (LCST) phase transition upon heating, which can be directly observed from the appearance of TRPIL solution at 25 and 60 o

C (Figure S2). Moreover, the turbidity measurement of TRPIL solution with a certain

concentration of 10 wt% shows a cloud point of around 58 oC in the heating process. The resultant TRPIL was further employed as a polymer surfactant to exfoliate three typical TDMs including MoS2, graphite and h-BN into single- or few-layered nanosheets in aqueous medium with the aid of vigorous sonication. Considering the possible phase transition of TRPIL at high temperature, the sonication treatment was conducted below 25 oC throughout the whole process. Figure 2a shows the photograph of MoS2, graphite and h-BN suspensions in water before and after TRPIL addition. TDM powders all show a severe aggregation in water due to their strong hydrophobicity. Nevertheless, incorporation of slight TRPIL can dramatically facilitate their wetting and distribution in water, which is an important precondition for subsequent exfoliation. After TRPIL-assisted sonication treatment, the exfoliated nanosheets are separated by centrifugation and the collected nanosheets show a good dispersibility in water even after seven days (Figure 2b). Among them, the exfoliation of MoS2 always experiences an obvious color change from black to deep green, while the 8 ACS Paragon Plus Environment

Page 8 of 30

Page 9 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


exfoliated BNNSs aqueous suspension exhibits a milky color compared with the white color of h-BN. The concentration of exfoliated nanosheets was estimated by filtration and weighing method as used by other researchers.39,

41

The results show that the concentration of

exfoliated MoS2, graphene and BNNS in suspensions was found to be 1.24, 0.48 and 0.43 mg/ml with the corresponding dispersion yield of 24.8, 9.6 and 8.6 %, respectively. When this exfoliation processing was scaled up to ten times, a significant dispersion yield of exfoliated nanosheets can still be maintained, suggesting the good scalability of this method (Figure S3). To ascertain the morphology of exfoliated nanosheets, transmission electron microscopy (TEM) analysis was performed. As shown in Figure 2c-e, the exfoliated MoS2 nanosheets show a lateral size of 100-500 nm with relatively rigid feature. However, the exfoliated graphene nanosheets and BNNSs both show a curled morphology due to their ultrathin feature. The high-resolution TEM (HRTEM) images further reveal the edge of an exfoliated MoS2 nanosheet with 4-5 layers, graphene with 6-7 layers and BNNS with 5-6 layers, suggesting the successful exfoliation of TDMs (Figure 2f-h). The inset selected area electron diffraction (SAED) patterns of them confirm the single crystalline nature of hexagonal symmetry structure, suggesting that the lattice structure of the obtained nanosheets were hardly damaged during sonication. In addition, one can see some amorphous component tightly anchored on the nanosheets. This component is believed to be the noncovalent functionalized TRPIL on the exfoliated nanosheets by hydrophobic and cation-mediated interactions, which is favorable to stabilize the nanosheets in water. The exfoliation of these three typical TDMs is further confirmed by the scanning electron micrograph (SEM) image in Figure S4. Moreover, the dependent concentrations of the exfoliated nanosheets on the 9 ACS Paragon Plus Environment


feeding amount of TDMs are further studied at a fixed TRPIL of 2 mg/ml. Figure S5 shows that the exfoliated amount of nanosheets increase with the increase of TDMs up to a certain amount, and then decrease with the further increase of TDMs amount. These results indicate that the exfoliation process is highly dependent on the ratio of TRPIL/TDMs used. Additionally, it was found that TRPIL can be further extended to assist the reliable exfoliation of WS2 and Bi2Te3 in water (Figure S6).

Figure 2. (a) Photographs of the dispersion of layered material powders in water before and after TRPIL addition. (b) Photographs of the dispersed stability of exfoliated MoS2, BNNS and graphene in water. TEM images of exfoliated MoS2 (c), graphene (d) and BNNS (e); Insets show the corresponding dilute suspensions of the exfoliated nanosheet in water. High-resolution TEM images of exfoliated MoS2 (f), graphene (g) and BNNS (h); Insets show the corresponding SAED patterns.


Page 10 of 30

Page 11 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


Figure 3. AFM images and the corresponding height profile of exfoliated MoS2 (a and d), graphene (b and e) and BNNS (c and f). In view of the fact that the thickness information of the exfoliated nanosheet is unobservable in TEM and SEM characterization, thus atomic force microscopy (AFM) was further employed to study the structural feature. Figure 3a-c show the AFM morphology of the exfoliated MoS2, graphene and BNNS, respectively, and the corresponding height profiles are provided in Figure 3d-f. One can see that the thickness of MoS2 nanosheets is ~2 nm in comparison with ~2.5 nm of graphene and ~3 nm of BNNS. In spite of the noncovalent functionalization of TRPIL on the exfoliated nanosheets, the largely decreased thickness suggests that TRPIL-assisted sonication can reliably peel off various 2D layered materials into few-layered nanosheets in aqueous medium. In order to confirm the noncovalent functionalization of exfoliated nanosheets by TRPIL, the elemental compositions and the bonding modes of these exfoliated nanosheets were investigated by X-ray photoelectron spectroscopy (XPS) measurement. As expected, S and P elements that mainly derived from TRPIL are both detected in the exfoliated MoS2, graphene and BNNS, indicating the presence 11 ACS Paragon Plus Environment


of TRPIL on these nanosheets (Figure 4a-c). Further evidence of the adsorbed TRPIL on exfoliated nanosheets was determined by TGA analysis in a N2 atmosphere. As shown in Figure S7a-c, the TGA curves of exfoliated nanosheets all show an enhanced weight loss compared with those of the pristine TDMs due to the thermal decomposition of physically adsorbed TRPIL. Moreover, the high-resolution XPS spectra of Mo 3d of MoS2, C 1s of graphene and B 1s of BNNS were also investigated. Two main intense peaks located at 227.4 and 230.5 eV correspond to the Mo 3d5/2 and Mo 3d3/2 of MoS2, respectively (Figure 4d).42 No obvious trace of Mo6+ 3d5/2 peak is observed, indicating the absence of oxidation during the sonication treatment.42 The C 1s region of graphene nanosheet can be deconvoluted into two peaks that assigned to C-C/C=C and C-S(P) (Figure 4e).43-44 Herein, the detected C-S(P) bond is derived from the TRPIL. Additionally, B 1s region of exfoliated BNNS only shows a single peak at 188.7 eV (Figure 4f).

Figure 4. XPS survey spectra of exfoliated MoS2 (a), graphene (b) and BNNS (c). High-resolution Mo 3d XPS spectrum of exfoliated MoS2 (d), C 1s XPS spectrum of exfoliated graphene (e) and B 1s XPS spectrum of exfoliated BNNS (f).


Page 12 of 30

Page 13 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


The crystalline nature of these TDMs before and after exfoliation was systematically studied by X-ray diffraction (XRD). As shown in Figure 5a, the XRD pattern of bulk MoS2 shows many strong, sharp peaks at the degree range from 10 to 70o, which match well to the hexagonal 2H-MoS2 (JCPDS: 65-1951).20 After sonication treatment, the peak of (002) reflection at 14.4o broadens accompanied with the disappearance of most of the other diffraction peaks, indicating the high exfoliation feature of MoS2 nanosheets. For the raw graphite, there are two main diffraction peaks at 26.9o and 54.8o, which corresponds to graphitic (002) and (004), respectively (Figure 5b). After TRPIL-assisted exfoliation, the peaks of the as-exfoliated graphene become very weak though the amount of the sample for testing is the same as that of the raw graphite, suggesting that majority of the sample has been peeled off into few-layered nanosheets. Similarly, the decreased intensity of diffraction peaks at 42.1o, 44.3o and 55.6o for the exfoliated BNNSs compared with bulk h-BN indicates the formation of ultrathin BNNS after sonication (Figure 5c).45 In addition, the enlarged (002) diffraction peaks, that originates from the interlayer distance between sheets, of these three TDMs before and after exfoliation are displayed in Figure 5d-f. One can clearly see that the (002) diffraction peaks of the exfoliated nanosheets all show a slight downshift in comparison with that of their pristine materials, suggesting the enhanced interlayer distance of exfoliated nanosheets due to the intercalation of TRPIL. The downshift of (002) diffraction peak has also been widely observed for the exfoliated nanosheets in the previously reported works.45-47



Figure 5. XRD patterns and the corresponding enlarged XRD patterns of the layered materials before and after exfoliation: (a, d) MoS2, (b, e) graphene and (c, f) BNNS. Raman analysis, as a universal and facile measurement for 2D layered materials, was also used to confirm the successful exfoliation of MoS2 and graphite. Figure 6a shows the typical Raman spectrum of bulk MoS2 with two main modes, A1g (stretching of the sulfur atoms) and E2g1 (in-plane bending), located at 395 and 367 cm-1, respectively.48 These two characteristic peaks are both red-shifted after exfoliation, indicating the successful fabrication of few-layered MoS2 nanosheets.15 Moreover, it is believed that the adsorption of TRPIL on the exfoliated MoS2 nanosheets may mediate the atomic bending vibration, resulting in the shift of E2g1 mode.14 For graphite, three characteristic peaks of D band (associating with disordered carbon, defects and edges) at 1341 cm-1, G band (corresponding to the ordered sp2-bonded carbon) at 1568 cm-1 and 2D band at 2695 cm-1 were detected in the Raman spectrum (Figure 6b).49 As expected, the Raman spectrum of exfoliated graphene reveals a noticeable change that the 2D and G bands both slightly blue-shift due to the strong interaction between graphene nanosheet and TRPIL chains, which has also been widely reported in many similar works.43, 49-50 Additionally, the ratio of ID/IG increases from 0.11 of the natural graphite to 14 ACS Paragon Plus Environment

Page 14 of 30

Page 15 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


0.22 of the exfoliated graphene, indicating that the prepared graphene nanosheets have a high quality while the enhancement of ID/IG value is mainly originated from the defects at the edges of relatively smaller flake.51 Among these TDMs, exfoliation of MoS2 is the most widely-investigated one in recent years. Therefore, UV-Vis spectroscopy, another commonly used technique, was further used to confirm the exfoliation of bulk MoS2. As shown in Figure 6c, the absorption spectrum of bulk MoS2 show featureless absorbance at the wavelength range from 300 to 800 nm. While for the exfoliated MoS2 nanosheet, two characteristic absorption peaks at 613 and 671 nm corresponding to the direct transition of the valance band to the conduction band at the K-point of the Brillouin zone are presented, suggesting the few-layered feature of exfoliated MoS2.29 Moreover, the absorption intensity of these characteristic peaks can also qualitatively reflect the concentration of exfoliated MoS2 nanosheets in suspension.14,

30

Thus, the

influence of TRPIL concentration on the final exfoliation yield of MoS2 nanosheet was investigated by UV-vis measurement. Figure 6d shows the absorption spectra of dilute MoS2 suspension that exfoliated in aqueous solution with different TRPIL concentrations. It can be seen that increasing TRPIL concentration facilitates the exfoliation of MoS2 and the largest yield of MoS2 nanosheet is delivered at the TRPIL concentration of 2 mg/ml. However, further increase of TRPIL concentration inversely leads to the decreased exfoliation yield. For comparison, other familiar water-soluble polymers, such as polyvinylpyrrolidone (PVP), poly(acrylic acid) (PAA) and poly(vinyl alcohol) (PVA), were also selected to exfoliate the bulk MoS2 under the identical conditions. As shown in Figure S8, MoS2 exfoliated by TRPIL shows a stronger absorption than that exfoliated by PAA, PVP and PVA, suggesting the more 15 ACS Paragon Plus Environment


reliable exfoliation of MoS2 in TRPIL than other three water-soluble polymers. The effective exfoliation of these TDMs in TRPIL aqueous solution can be attributed to the following factors. Guan et al. reported that the nonpolar benzene rings in bovine serum albumin (BSA) tend to firmly anchor on the MoS2 surface to facilitate their uniform dispersion and effective exfoliation in aqueous solution.14 Similarly, the hydrophobic chain backbone and the cationic [P4,4,4,4] moiety in TRPIL can be adhered to the TDMs to improve their homogeneous dispersion in water. During the subsequent sonication, it is assumed that the cationic [P4,4,4,4] moiety of TRPIL first intercalates into the galleries of 2D layered materials via a strong cation-π interaction to weaken the inherent van der Waals (VDWs) force between adjacent sheets, promoting the easier exfoliation of 2D layered materials under vigorous sonication.39, 52

Moreover, the anionic poly(vinyl sulfonate) moiety acts as a polymer surfactant to stabilize

the detached nanosheets.

Figure 6. Raman spectra of MoS2 (a) and graphene (b) before and after exfoliation. (c) UV-visible absorption spectra of MoS2 before and after exfoliation. (d) UV-visible spectra of MoS2-TRPIL suspensions exfoliated under different TRPIL concentrations after diluting by 10 times. (e) Reversibly thermo-responsive dispersion stability of exfoliated MoS2, graphene and BNNS in TRPIL aqueous suspensions. 16 ACS Paragon Plus Environment

Page 16 of 30

Page 17 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


It is well known that the LCST-type thermo-responsive polymers would undergo phase separation above their LCST temperatures. Herein, the temperature-sensitive phase transition behavior of TRPIL has been used to reversibly control the distribution of the exfoliated MoS2, graphene and BN nanosheets in water via regulating the temperature below or above LCST temperature. As shown in Figure 6e, when the temperature of exfoliated nanosheet/TRPIL suspensions increase to 65 oC, the exfoliated nanosheets would precipitate at the bottom of the vial due to the phase transition induced agglomeration of TRPIL-modified nanosheets.53 As the temperature returns to 25 oC, simple shake will be able to achieve the re-dispersion of these nanosheets in water. TRPIL-assisted LPE technique in this work enables to achieve the stable dispersion of non-covalently functionalized nanosheets in water, thus the resultant dispersions are very favorable for the preparation of nanocomposite hydrogel which requires the nanofillers to be highly hydrophilic and well soluble in aqueous media. Recently, smart hydrogels, whose appearance can response to the environmental stimuli (e.g., temperature, PH, electricity and light, etc.), have attracted large interest by researchers.54-56 In this work, to combine the photothermal effect of MoS2 and temperature-sensitive feature of TRPIL for smart nanocomposite hydrogel, the obtained TRPIL-functionalized MoS2 was homogeneously incorporated into TRPIL hydrogel owing to their high hydrophilicity and thin lamellar structure through the in-situ polymerization of additionally added IL monomer. The MoS2 nanosheets with noncovalent functionalization of TRPIL are expected to have a good miscibility with the hydrogel matrix. As shown in Figure 7a, the neat TRPIL hydrogel is colorless and transparent while the TRPIL/MoS2 composite hydrogel shows a uniform deep 17 ACS Paragon Plus Environment


green color which originated from MoS2 nanosheet, suggesting the successful incorporation of MoS2 into TRPIL hydrogel. After separation from the mold, the TRPIL hydrogel and TRPIL/MoS2 composite hydrogel both show free-standing feature (Figure S9). However, the TRPIL/MoS2 composite hydrogel shows a robust property and good repeated compression and recovery performance (Movie S1). More importantly, the thermo-responsiveness of TRPIL can also endow the synthesized TRPIL/MoS2 composite hydrogel with reversibly temperature-responsive color change. As shown in Figure 7b, the appearance of TRPIL/MoS2 hydrogel changes from deep green to offwhite at 60 oC within 2 min due to the phase separation of TRPIL above its LCST temperature. It should be noted that the phase separation of TRPIL does not cause a perceptible volume change of the TRPIL/MoS2 hydrogel which is commonly observed in many temperature-sensitive hydrogels based on thermo-responsive polymers.57-59 Upon removal of the heating source, the composite hydrogel would gradually return to the initial state in a short period within 2 min (Figure S10). On the other hand, owing to the high near-infrared (NIR) absorbance of MoS2, it has been reported that MoS2 possesses a strong photo-thermal conversion effect, which has been extensively used in NIR photo-thermal therapy of cancer and stimuli-responsive smart materials.26, 60-61 Therefore, the NIR photo-responsibility of TRPIL/MoS2 hydrogel had been also studied. One can note that exposure of TRPIL/MoS2 hydrogel under NIR laser can also rapidly trigger the phase separation of TRPIL and then induce the color change from deep green to offwhite within 5 seconds (Figure 7c). However, the NIR irradiation treatment on neat TRPIL hydrogel cannot cause the phase separation of TRPIL, indicating the strong photo-thermal transition effect of MoS2 in TRPIL/MoS2 hydrogel (Figure S11). Based on the dual photo- and 18 ACS Paragon Plus Environment

Page 18 of 30

Page 19 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


thermo-responsive color change of TRPIL/MoS2 hydrogel, we think that this smart hydrogel could be potentially encapsulated in textile for military camouflage through the reversible color change driven by temperature or NIR irradiation (Figure 7d). At relatively low temperature or no NIR irradiation, this smart textile shows green color, which is suitable for the camouflage in jungle or steppe. While at relatively high temperature or NIR irradiation, this smart textile exhibits offwhite color for the effective camouflage in desert or highway. Additionally, the TRPIL/MoS2 hydrogel is able to withstand a large-deformation compression with the compressive strength of above 0.12 MPa at 75 % strain (Figure 7e-f). While for the neat TRPIL hydrogel, as the deformation degree reaches 75 % strain, the compressive strength is about 0.03 MPa along with serious structure damage, suggesting the remarkable reinforcing effect of MoS2 nanosheets for the TRPIL hydrogel. When the external loading is removed, the TRPIL/MoS2 hydrogel can rapidly recover without any obvious damage (Figure 7e). To reveal the origin of good flexibility of TRPIL/MoS2 composite hydrogel, the hydrogel was fast frozen in liquid nitrogen and then freeze-dried for visualized FESEM characterization. As shown in Figure 7g, it exhibits a compact 3D porous framework and no apparent MoS2 aggregation can be observed. Moreover, the element mapping images also show the homogeneous distribution of P, S and Mo elements in the hydrogel (Figure S12). On the contrary, the freeze-dried TRPIL hydrogel shows a relatively collapsed pore structure but homogeneous distribution of S and P elements, indicating the significant reinforcing effect of MoS2 nanosheets on the pore wall (Figure S13). As reference, polyacrylamide (PAAm) hydrogel, one of the most familiar hydrogels, was also synthesized and characterized. The SEM image of freeze-dried PAAm hydrogel shows a hierarchical pore structure (Figure S14). 19 ACS Paragon Plus Environment


The difference in pore structure between PAAm and TRPIL hydrogel might be attributed to the different charge of these two polymers. Mechanical stability of the TRPIL/MoS2 composite hydrogel was further investigated by the representative loading-unloading compression (Figure 7h). The loading-unloading curve of the first compressive cycle reveals a noticeable hysteresis loop with a residual strain, suggesting a typical elastomer-like behavior. The following loading-unloading cycles show a slight decrease of the compressive strength and the hysteresis loop, suggesting the good elasticity and mechanical stability of the TRPIL/MoS2 hydrogel. This result also indicates that some of the cross-links might have been broken during the repeated compression, resulting in a weaker hydrogel.

Figure 7. (a) Photographs of the synthesized TRPIL hydrogel and MoS2/TRPIL composite hydrogel. (b) Photographs of the apparent color of MoS2/TRPIL composite hydrogel at 25 and 60 oC. (c) Photographs of the apparent color of MoS2/TRPIL composite hydrogel supported on a green leaf under no irradiation and 808 nm irradiation for 60 s. (d) Schematic illustration of the dual photo- and thermo-responsive TRPIL/MoS2 composite hydrogel. (e) Compression and recovery of TRPIL/MoS2 composite hydrogel. (f) Compressive stress-strain curves of TRPIL hydrogel and TRPIL/MoS2 composite hydrogel. (g) SEM image of the TRPIL/MoS2 composite hydrogel after freeze-drying. (h) Representative loading-unloading compression curves of TRPIL/MoS2 composite hydrogel.


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


Polymer-based nanocomposite films with large anisotropic thermal conductivity are highly needed in the next-generation portable and flexible electronic devices due to the requirement of flexibility, lightweight and robustness.62 BNNS, as one kind of commonly-used thermally conductive fillers, has a large in-plane thermal conductivity of 2000 W m-1 K-1, thus, the well-ordered alignment of them is very important to obtain an exciting performance.63 Herein, the exfoliated BNNSs with noncovalent functionalization of TRPIL were used to prepare the nacre-like BNNS/PVA film based on aqueous suspension of exfoliated BNNS and trace amount of PVA via a vacuum-filtration-assisted self-assembly technique. During the filtration, the vacuum pressure would direct the layered BNNSs to adjust their basal plane parallel to the surface and eventually form the typical “brick-and-mortar” hierarchical structure of BNNSs and PVA.64-65 The obtained BNNS/PVA film shows a smooth surface and excellent flexibility (Figure 8a-c). FESEM was used to characterize the cross-sectional microstructure of the hybrid film. As shown in Figure 8d-e, the hybrid film shows a well-packed and nacre-like layered structure with a highly in-plane orientation of BNNSs. Owing to the hydrogen bond and cation-π interaction between PVA and TRPIL-functionalized BNNSs, PVA, as the “mortar”, is expected to crosslink the neighboring BNNSs to improve the mechanical performance and meanwhile reduce the interface thermal resistance of the hybrid film (Figure 8f).65 Therefore, the thermal transport and mechanical properties of the BNNS/PVA film with various PVA amounts were further investigated. Herein, the loading of PVA in hybrid films was estimated by TGA measurement under N2 (Figure S15). Figure 8g depicts the typical stress-strain curves of the BNNS/PVA film with different PVA amount. It can be seen that the tensile strength of the hybrid film increases with the PVA loading, which 21 ACS Paragon Plus Environment


might be attributed to the enhanced gluing effect between BNNSs caused by the increasing H-bonds. The hybrid films incorporated with 7.5 and 9.2 wt% PVA show a high tensile strength of ~75 and ~80 MPa, respectively, suggesting a good mechanical property. In addition, BNNS recently has been regarded as a promising candidate in the thermally conductive composite due to their high thermal conductivity and excellent electrical insulation.62-63,

66

Therefore, we further studied the in-plane thermal conductivity of the

BNNS/PVA hybrid films. As shown in Figure 8h, the thermal conductivity of hybrid film also increases with the PVA loading. Although PVA has a very low intrinsic thermal conductivity, it would fill the interval between adjacent BNNSs and bridge them with strong H-bonding interaction, resulting in the great decrease of interface thermal resistance.65, 67 At a PVA loading of 7.5 wt%, a largest thermal conductivity of 12.2 W/mK is obtained. This value is largely higher than that of the similar BNNS-PVA paper reported by Xu et al., which might be attributed to the more ordered alignment and larger size of BNNS in our work.65 Further increase of PVA concentration conversely causes the decrease of thermal conductivity. That’s because the excess PVA molecules without interacting with BNNS would increase the interval of neighboring BNNSs and then lead to a larger phonon scattering. The high in-plane thermal conductivity, excellent mechanical flexibility and strength of BNNS/PVA film could render them promisingly used in various flexible electronic devices to dissipate heat from the hot regions along the in-plane direction.


Page 22 of 30

Page 23 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


Figure 8. (a-c) Optical photographs and (d-e) Cross-sectional SEM image of of BNNS/PVA film; (f) Schematic illustration of the nacre-like microstructure and the interactions in BNNS/PVA film; (g) Stress-strain curves and (h) in-plane thermal conductivity of the BNNS/PVA film with different PVA loadings.

3. Conclusions In summary, a synthesized thermo-responsive polymeric ionic liquid (TRPIL) was first employed to assist the direct exfoliation of bulk MoS2, graphite and h-BN into single- and few-layer nanosheets in an aqueous medium. During the sonication, TRPIL chains would partially adsorb onto the 2D layered materials by hydrophobic or cation-mediated interactions to facilitate their exfoliation and non-covalent functionalization. As a result, a high dispersion yield of exfoliated nanosheets was obtained and these aqueous suspensions all show reversible temperature-mediated distribution stability due to the thermo-responsive phase transition behavior of TRPIL. As a proof of potential applications, the TRPIL-functionalized MoS2 nanosheets were homogeneously incorporated into TRPIL hydrogel to prepare the TRPIL/MoS2 composite hydrogel with temperature and photo dual-responsive color change. Besides, artificial nacre-like BNNS/PVA films with highly thermally conductive property were also fabricated through a vacuum-filtration-induced layer-by-layer self-assembly of the 23 ACS Paragon Plus Environment


TRPIL-functionalized BNNSs. ASSOCIATED CONTENT Supporting Information Available. Synthesis of TRPIL, liquid exfoliation of various 2D materials, preparation of TRPIL/MoS2 hydrogel and BNNS/PVA film, 1H NMR spectra of IL monomer and TRPIL, turbidity curve of TRPIL aqueous solution, SEM images and TGA curves of the exfoliated MoS2, graphene and BNNS, UV-visible spectra of the exfoliated MoS2 suspension, optical image of TRPIL and TRPIL/MoS2 hydrogels, element mapping images of TRPIL/MoS2 hydrogel after free-drying and TGA curves of BNNS/PVA films.

AUTHOR INFORMATION Corresponding Author *Authors for Correspondence: [email protected] ORCID Peiyi Wu: 0000-0001-7235-210X Notes The authors declare no competing financial interest. ACKNOWLEDGMENTS We gratefully acknowledge the financial support from the National Science Foundation of China (Nos. 51733003, 21674025, 51473038).

Reference 1.

Coleman, J. N.; Lotya, M.; O'Neill, A.; Bergin, S. D.; King, P. J.; Khan, U.; Young, K.; Gaucher, A.; De, S.; Smith, R. J.; Shvets, I. V.; Arora, S. K.; Stanton, G.; Kim, H.-Y.; Lee, K.; Kim, G. T.; Duesberg, G. S.; Hallam, T.; Boland, J. J.; Wang, J. J.; Donegan, J. F.; Grunlan, J. C.; Moriarty, G.; Shmeliov, A.; Nicholls, R. J.; Perkins, J. M.; Grieveson, E. M.; Theuwissen, K.; McComb, D. W.; Nellist, P. D.; Nicolosi, V., Two-Dimensional Nanosheets Produced by Liquid Exfoliation of Layered Materials. Science 2011, 331, 568-571. 24 ACS Paragon Plus Environment

Page 24 of 30

Page 25 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


2.

3. 4.

5.

6.

7. 8.

9. 10.

11.

12.

13. 14.

15.

16.

Xu, S.; Li, D.; Wu, P., One-Pot, Facile, and Versatile Synthesis of Monolayer MoS2/WS2 Quantum Dots as Bioimaging Probes and Efficient Electrocatalysts for Hydrogen Evolution Reaction. Adv. Funct Mater. 2015, 25, 1127-1136. Chen, Y.; Tan, C.; Zhang, H.; Wang, L., Two-dimensional graphene analogues for biomedical applications. Chem. Soc. Rev. 2015, 44, 2681-2701. Wang, Q. H.; Kalantar-Zadeh, K.; Kis, A.; Coleman, J. N.; Strano, M. S., Electronics and optoelectronics of two-dimensional transition metal dichalcogenides. Nat. Nanotechno. 2012, 7, 699-712. Koppens, F. H. L.; Mueller, T.; Avouris, P.; Ferrari, A. C.; Vitiello, M. S.; Polini, M., Photodetectors based on graphene, other two-dimensional materials and hybrid systems. Nat. Nanotechno. 2014, 9, 780-793. Luo, W.; Wang, Y.; Hitz, E.; Lin, Y.; Yang, B.; Hu, L., Solution Processed Boron Nitride Nanosheets: Synthesis, Assemblies and Emerging Applications. Adv. Funct. Mater. 2017, 27, 1701450. Wang, X.; Feng, H.; Wu, Y.; Jiao, L., Controlled Synthesis of Highly Crystalline MoS2 Flakes by Chemical Vapor Deposition. J. Am. Chem. Soc. 2013, 135, 5304-5307. Xing, W.; Chen, Y.; Wu, X.; Xu, X.; Ye, P.; Zhu, T.; Guo, Q.; Yang, L.; Li, W.; Huang, H., PEDOT:PSS-Assisted Exfoliation and Functionalization of 2D Nanosheets for High-Performance Organic Solar Cells. Adv. Funct. Mater. 2017, 27, 1701622. Yi, M.; Shen, Z., A review on mechanical exfoliation for the scalable production of graphene. J. Mater. Chem. A 2015, 3, 11700-11715. 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. Nanotechno. 2008, 3, 563-568. 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, 12, 272-293. Tao, H.; Zhang, Y.; Gao, Y.; Sun, Z.; Yan, C.; Texter, J., Scalable exfoliation and dispersion of two-dimensional materials - an update. Phys. Chem. Chem. Phys. 2017, 19, 921-960. Chen, X.; Dobson, J. F.; Raston, C. L., Vortex fluidic exfoliation of graphite and boron nitride. Chem. Commun. 2012, 48, 3703-3705. Liu, W.; Zhao, C.; Zhou, R.; Zhou, D.; Liu, Z.; Lu, X., Lignin-assisted exfoliation of molybdenum disulfide in aqueous media and its application in lithium ion batteries. Nanoscale 2015, 7, 9919-9926. Ravula, S.; Essner, J. B.; Baker, G. A., Kitchen-Inspired Nanochemistry: Dispersion, Exfoliation, and Hybridization of Functional MoS2 Nanosheets Using Culinary Hydrocolloids. Chemnanomat 2015, 1, 167-177. 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, 23, 3944. 25 ACS Paragon Plus Environment


17. Liu, J.; Zeng, Z.; Cao, X.; Lu, G.; Wang, L.-H.; Fan, Q.-L.; Huang, W.; Zhang, H., Preparation of MoS2-Polyvinylpyrrolidone Nanocomposites for Flexible Nonvolatile Rewritable Memory Devices with Reduced Graphene Oxide Electrodes. Small 2012, 8, 3517-3522. 18. Lee, D.; Lee, B.; Park, K. H.; Ryu, H. J.; Jeon, S.; Hong, S. H., Scalable Exfoliation Process for Highly Soluble Boron Nitride Nanoplatelets by Hydroxide-Assisted Ball Milling. Nano Lett. 2015, 15, 1238-1244. 19. Morishita, T.; Okamoto, H., Facile Exfoliation and Noncovalent Superacid Functionalization of Boron Nitride Nanosheets and Their Use for Highly Thermally Conductive and Electrically Insulating Polymer Nanocomposites. ACS Appl. Mater. Interfaces 2016, 8, 27064-27073. 20. Feng, X.; Xing, W.; Yang, H.; Yuan, B.; Song, L.; Hu, Y.; Liew, K. M., High-Performance Poly(ethylene oxide)/Molybdenum Disulfide Nanocomposite Films: Reinforcement of Properties Based on the Gradient Interface Effect. ACS Appl. Mater. Interfaces 2015, 7, 13164-13173. 21. Li, Y.; Zhu, H.; Shen, F.; Wan, J.; Lacey, S.; Fang, Z.; Dai, H.; Hu, L., Nanocellulose as green dispersant for two-dimensional energy materials. Nano Energy 2015, 13, 346-354. 22. Khan, U.; May, P.; O'Neill, A.; Bell, A. P.; Boussac, E.; Martin, A.; Semple, J.; Coleman, J. N., Polymer reinforcement using liquid-exfoliated boron nitride nanosheets. Nanoscale 2013, 5, 581-587. 23. Bourlinos, A. B.; Georgakilas, V.; Zboril, R.; Steriotis, T. A.; Stubos, A. K.; Trapalis, C., Aqueous-phase exfoliation of graphite in the presence of polyvinylpyrrolidone for the production of water-soluble graphenes. Solid State Commun. 2009, 149, 2172-2176. 24. Vega-Mayoral, V.; Backes, C.; Hanlon, D.; Khan, U.; Gholamvand, Z.; O'Brien, M.; Duesberg, G. S.; Gadermaier, C.; Coleman, J. N., Photoluminescence from Liquid-Exfoliated WS2 Monomers in Poly(Vinyl Alcohol) Polymer Composites. Adv. Funct. Mater. 2016, 26, 1028-1039. 25. 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. 26. Zong, L.; Li, M.; Li, C., Bioinspired Coupling of Inorganic Layered Nanomaterials with Marine Polysaccharides for Efficient Aqueous Exfoliation and Smart Actuating Hybrids. Adv. Mater. 2017, 29, 1604691. 27. Yuan, Y.; Li, R.; Liu, Z., Establishing Water-Soluble Layered WS2 Nanosheet as a Platform for Biosensing. Anal. Chem. 2014, 86, 3610-3615. 28. Lu, F.; Wang, F.; Gao, W.; Huang, X.; Zhang, X.; Li, Y., Aqueous soluble boron nitride nanosheets via anionic compound-assisted exfoliation. Mater. Express 2013, 3, 144-150. 29. Wang, D.; Song, L.; Zhou, K.; Yu, X.; Hu, Y.; Wang, J., Anomalous nano-barrier effects of ultrathin molybdenum disulfide nanosheets for improving the flame retardance of polymer nanocomposites. J. Mater. Chem. A 2015, 3, 14307-14317. 30. Guan, G.; Zhang, S.; Liu, S.; Cai, Y.; Low, M.; Teng, C. P.; Phang, I. Y.; Cheng, Y.; Duei, K. L.; Srinivasan, B. M.; Zheng, Y.; Zhang, Y.-W.; Han, M.-Y., Protein Induces Layer-by-Layer Exfoliation of Transition Metal Dichalcogenides. J. Am. Chem. Soc. 26 ACS Paragon Plus Environment

Page 26 of 30

Page 27 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


31.

32. 33.

34.

35.

36.

37. 38. 39.

40.

41.

42.

43.

44.

2015, 137, 6152-6155. Ayan-Varela, M.; Perez-Vidal, O.; Paredes, J. I.; Munuera, J. M.; Villar-Rodil, S.; Diaz-Gonzalez, M.; Fernandez-Sanchez, C.; Silva, V. S.; Cicuendez, M.; Vila, M.; Martinez-Alonso, A.; Tascon, J. M. D., Aqueous Exfoliation of Transition Metal Dichalcogenides Assisted by DNA/RNA Nucleotides: Catalytically Active and Biocompatible Nanosheets Stabilized by Acid-Base Interactions. ACS Appl. Mater. Interfaces 2017, 9, 2835-2845. Rogers, R. D., Materials science - Reflections on ionic liquids. Nature 2007, 447, 917-918. Li, W.; Wu, P., Unusual thermal phase transition behavior of an ionic liquid and poly(ionic liquid) in water with significantly different LCST and dynamic mechanism. Polym. Chem. 2014, 5, 5578-5590. Morishita, T.; Okamoto, H.; Katagiri, Y.; Matsushita, M.; Fukumori, K., A high-yield ionic liquid-promoted synthesis of boron nitride nanosheets by direct exfoliation. Chem. Commun. 2015, 51, 12068-12071. Ravula, S.; Baker, S. N.; Kamath, G.; Baker, G. A., Ionic liquid-assisted exfoliation and dispersion: stripping graphene and its two-dimensional layered inorganic counterparts of their inhibitions. Nanoscale 2015, 7, 4338-4353. Matsumoto, M.; Saito, Y.; Park, C.; Fukushima, T.; Aida, T., Ultrahigh-throughput exfoliation of graphite into pristine 'single-layer' graphene using microwaves and molecularly engineered ionic liquids. Nat. Chem. 2015, 7, 730-736. Thi Phuong Thuy, P.; Cho, C.-W.; Yun, Y.-S., Environmental fate and toxicity of ionic liquids: A review. Water Res. 2010, 44, 352-372. Mecerreyes, D., Polymeric ionic liquids: Broadening the properties and applications of polyelectrolytes. Prog. Polym. Sci. 2011, 36, 1629-1648. Biswas, Y.; Dule, M.; Mandal, T. K., Poly(ionic liquid)-Promoted Solvent-Borne Efficient Exfoliation of MoS2/MoSe2 Nanosheets for Dual-Responsive Dispersion and Polymer Nanocomposites. J. Phys. Chem. C 2017, 121, 4747-4759. Lei, Z.; Zhou, Y.; Wu, P., Simultaneous Exfoliation and Functionalization of MoSe2 Nanosheets to Prepare "Smart" Nanocomposite Hydrogels with Tunable Dual Stimuli-Responsive Behavior. Small 2016, 12, 3112-3118. Kapil, N.; Singh, A.; Singh, M.; Das, D., Efficient MoS2 Exfoliation by Cross-beta-Amyloid Nanotubes for Multistimuli-Responsive and Biodegradable Aqueous Dispersions. Angew. Chem. Int. Ed. 2016, 55, 7772-7776. Ghasemi, F.; Mohajerzadeh, S., Sequential Solvent Exchange Method for Controlled Exfoliation of MoS2 Suitable for Phototransistor Fabrication. ACS Appl. Mater. Interfaces 2016, 8, 31179-31191. Lee, J. W.; Lee, J. U.; Jo, J. W.; Bae, S.; Kim, K. T.; Jo, W. H., In-situ preparation of graphene/poly(styrenesulfonic acid-graft-polyaniline) nanocomposite via direct exfoliation of graphite for supercapacitor application. Carbon 2016, 105, 191-198. Wang, X.; Liu, Y.; Wu, P., Water-soluble triphenylphosphine-derived microgel as the template towards in-situ nitrogen, phosphorus co-doped mesoporous graphene framework for supercapacitor and electrocatalytic oxygen reduction. Chem. Eng. J. 2017, 328, 417-427. 27 ACS Paragon Plus Environment


45. Zhu, W.; Gao, X.; Li, Q.; Li, H.; Chao, Y.; Li, M.; Mahurin, S. M.; Li, H.; Zhu, H.; Dai, S., Controlled Gas Exfoliation of Boron Nitride into Few-Layered Nanosheets. Angew. Chem. Int. Ed. 2016, 55, 10766-10770. 46. Fan, X.; Xu, P.; Li, Y. C.; Zhou, D.; Sun, Y.; Nguyen, M. A. T.; Terrones, M.; Mallouk, T. E., Controlled Exfoliation of MoS2 Crystals into Trilayer Nanosheets. J. Am. Chem. Soc. 2016, 138, 5143-5149. 47. Peng, J.; Weng, J., One-pot solution-phase preparation of a MoS2/graphene oxide hybrid. Carbon 2015, 94, 568-576. 48. Quinn, M. D. J.; Ngoc Han, H.; Notley, S. M., Aqueous Dispersions of Exfoliated Molybdenum Disulfide for Use in Visible-Light Photocatalysis. ACS Appl. Mater. Interfaces 2013, 5, 12751-12756. 49. Xu, S.; Xu, Q.; Wang, N.; Chen, Z.; Tian, Q.; Yang, H.; Wang, K., Reverse-Micelle-Induced Exfoliation of Graphite into Graphene Nanosheets with Assistance of Supercritical CO2. Chem. Mater. 2015, 27, 3262-3272. 50. Viinikanoja, A.; Kauppila, J.; Damlin, P.; Makila, E.; Leiro, J.; Aaritalo, T.; Lukkari, J., Interactions between graphene sheets and ionic molecules used for the shear-assisted exfoliation of natural graphite. Carbon 2014, 68, 195-209. 51. Xu, L.; McGraw, J.-W.; Gao, F.; Grundy, M.; Ye, Z.; Gu, Z.; Shepherd, J. L., Production of High-Concentration Graphene Dispersions in Low-Boiling-Point Organic Solvents by Liquid-Phase Noncovalent Exfoliation of Graphite with a Hyperbranched Polyethylene and Formation of Graphene/Ethylene Copolymer Composites. J. Phys. Chem. C 2013, 117, 10730-10742. 52. Zhang, W.; Wang, Y.; Zhang, D.; Yu, S.; Zhu, W.; Wang, J.; Zheng, F.; Wang, S.; Wang, J., A one-step approach to the large-scale synthesis of functionalized MoS2 nanosheets by ionic liquid assisted grinding. Nanoscale 2015, 7, 10210-10217. 53. Men, Y.; Li, X.-H.; Antonietti, M.; Yuan, J., Poly(tetrabutylphosphonium 4-styrenesulfonate): a poly(ionic liquid) stabilizer for graphene being multi-responsive. Polym. Chem. 2012, 3, 871-873. 54. Lei, Z.; Zhu, W.; Xu, S.; Ding, J.; Wan, J.; Wu, P., Hydrophilic MoSe2 Nanosheets as Effective Photothermal Therapy Agents and Their Application in Smart Devices. ACS Appl. Mater. Interfaces 2016, 8, 20900-20908. 55. Takashima, Y.; Yonekura, K.; Koyanagi, K.; Iwaso, K.; Nakahata, M.; Yamaguchi, H.; Harada, A., Multifunctional Stimuli-Responsive Supramolecular Materials with Stretching, Coloring, and Self-Healing Properties Functionalized via Host-Guest Interactions. Macromolecules 2017, 50, 4144-4150. 56. Ma, C.; Le, X.; Tang, X.; He, J.; Xiao, P.; Zheng, J.; Xiao, H.; Lu, W.; Zhang, J.; Huang, Y.; Chen, T., A Multiresponsive Anisotropic Hydrogel with Macroscopic 3D Complex Deformations. Adv. Funct. Mater. 2016, 26, 8670-8676. 57. Han, L.; Zhang, Y.; Lu, X.; Wang, K.; Wang, Z.; Zhang, H., Polydopamine Nanoparticles Modulating Stimuli-Responsive PNIPAM Hydrogels with Cell/Tissue Adhesiveness. ACS Appl. Mater. Interfaces 2016, 8, 29088-29100. 58. Zhang, C.-L.; Cao, F.-H.; Wang, J.-L.; Yu, Z.-L.; Ge, J.; Lu, Y.; Wang, Z.-H.; Yu, S.-H., Highly Stimuli-Responsive Au Nanorods/Poly(N-isopropylacrylamide) (PNIPAM) Composite Hydrogel for Smart Switch. ACS Appl. Mater. Interfaces 2017, 9, 28 ACS Paragon Plus Environment

Page 28 of 30

Page 29 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


59.

60.

61.

62.

63.

64.

65.

66.

67.

24857-24863. Deguchi, Y.; Kohno, Y.; Ohno, H., Reversible water uptake/release by thermoresponsive polyelectrolyte hydrogels derived from ionic liquids. Chem. Commun. 2015, 51, 9287-9290. Liu, T.; Wang, C.; Gu, X.; Gong, H.; Cheng, L.; Shi, X.; Feng, L.; Sun, B.; Liu, Z., Drug Delivery with PEGylated MoS2 Nano-sheets for Combined Photothermal and Chemotherapy of Cancer. Adv. Mater. 2014, 26, 3433-3440. Yin, W.; Yan, L.; Yu, J.; Tian, G.; Zhou, L.; Zheng, X.; Zhang, X.; Yong, Y.; Li, J.; Gu, Z.; Zhao, Y., High-Throughput Synthesis of Single-Layer MoS2 Nanosheets as a Near-Infrared Photothermal-Triggered Drug Delivery for Effective Cancer Therapy. ACS Nano 2014, 8, 6922-6933. Wu, K.; Fang, J.; Ma, J.; Huang, R.; Chai, S.; Chen, F.; Fu, Q., Achieving a Collapsible, Strong, and Highly Thermally Conductive Film Based on Oriented Functionalized Boron Nitride Nanosheets and Cellulose Nanofiber. ACS Appl. Mater. interfaces 2017, 9, 30035-30045. Wang, X.; Wu, P., Preparation of Highly Thermally Conductive Polymer Composite at Low Filler Content via a Self-Assembly Process between Polystyrene Microspheres and Boron Nitride Nanosheets. ACS Appl. Mater. Interfaces 2017, 9, 19934-19944. Song, N.; Jiao, D.; Ding, P.; Cui, S.; Tang, S.; Shi, L., Anisotropic thermally conductive flexible films based on nanofibrillated cellulose and aligned graphene nanosheets. J. Mater. Chem. C 2016, 4, 305-314. Zeng, X.; Ye, L.; Yu, S.; Li, H.; Sun, R.; Xu, J.; Wong, C.-P., Artificial nacre-like papers based on noncovalent functionalized boron nitride nanosheets with excellent mechanical and thermally conductive properties. Nanoscale 2015, 7, 6774-6781. Chen, J.; Huang, X.; Zhu, Y.; Jiang, P., Cellulose Nanofiber Supported 3D Interconnected BN Nanosheets for Epoxy Nanocomposites with Ultrahigh Thermal Management Capability. Adv. Funct. Mater. 2017, 27, 1604754. Yao, Y.; Zeng, X.; Wang, F.; Sun, R.; Xu, J.-b.; Wong, C.-P., Significant Enhancement of Thermal Conductivity in Bioinspired Freestanding Boron Nitride Papers Filled with Graphene Oxide. Chem. Mater. 2016, 28, 1049-1057.

Table of contents 29 ACS Paragon Plus Environment



Page 30 of 30

Aqueous Phase Exfoliation of Two-dimensional Materials Assisted by

Recommend Documents