Two-Dimensional Nanosheets by Rapid and Efficient Microwave

Article Cite This: Chem. Mater. 2018, 30, 5932−5940

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Two-Dimensional Nanosheets by Rapid and Efficient Microwave Exfoliation of Layered Materials Wei Wu,† Jun Xu,† Xingwei Tang,† Peiwen Xie,† Xianghui Liu,† Jingsan Xu,‡ Han Zhou,*,† Di Zhang,† and Tongxiang Fan*,† †

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State Key Lab of Metal Matrix Composites, School of Materials Science and Engineering, Shanghai Jiao Tong University, Shanghai 200240, China ‡ School of Chemistry, Physics and Mechanical Engineering, Queensland University of Technology, Brisbane City, Queensland 4000, Australia S Supporting Information *

ABSTRACT: Layered materials beyond graphene have generated renewed interests in numerous fields. Liquid-phase exfoliation methods face essential challenges in their universal application toward various twodimensional materials (2DMs), short processing time, high yield, chemical stability, ultrathin thickness, and large lateral size of the nanosheets. To date, few reported methods are satisfactory in these requirements. We develop a general microwave-assisted, rapid (30 min), efficient (up to 50% yield), and potentially scalable approach to exfoliate 2DMs into mono- and few-layer nanosheets of superior chemical stability and large lateral size. 2DMs including h-BN, g-C3N4, BP, TMDs (MoS2, WS2, MoSe2), Zn2(bim)4, and Ti3C2Tx are tested for exfoliation in different fluid media including organic solvents and PF6−-containing ionic liquids (ILs). The nanosheets (e.g., BP) are surprisingly stable, probably attributed to solvation shells preventing the exfoliated sheets from reacting with water and oxygen. Theoretical simulations reveal that the dielectric constant of the fluid medium is a key factor determining the exfoliation efficiency. The preferred fluid media should be strongly polar (e.g., organic solvents with a high dielectric constant), which indicates materials’ ability to store electromagnetic energy via polarization. Finally, we demonstrate the 3D printing of nanosheet-based hybrids for potential applications. This general strategy paves a new promising pathway for the efficient liquid exfoliation of various 2DMs.

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INTRODUCTION Layered materials as two-dimensional (2D) systems have exotic electrical, optical, and chemical properties and various uses including catalysis,1−4 sensing,5,6 electronics,7,8 and energy storage/conversion,9,10 etc. Beyond graphene, many other two-dimensional materials (2DMs), including transition metal dichalcogenides (TMDs), hexagonal boron nitride (hBN), black phosphorus (BP), graphitic carbon nitride (gC3N4), and transition metal carbides/carbonitrides (MXenes), are also emerging as hot research fields.11−16 Liquid-phase exfoliation (LPE) proves the most common and useful method to cleave nanosheets from bulk 2DMs via meleclue/ion intercalation into interlayers and weakening the interlayer bonding,17,18 which can create ultrathin 2DMs in liquid media. Several significant methods have been developed, including sonication,18−21 shear force,22 ion intercalation,23 electrochemical exfoliation,24,25 and ion exchange.26 Sonicationassisted exfoliation is the most widely used approach and has been well-developed for a large variety of 2DMs exfoliation.18−21,27 Nevertheless, the sonication process is generally time-consuming (tens of hours), and the lateral sizes of the exfoliated 2DMs obviously decrease because a long-time © 2018 American Chemical Society

sonication force can break nanosheets as fragments. For lithium intercalation, it requires much time (e.g., 2−3 days) and careful operation to prevent destruction, particularly when using lithium compounds (e.g., LiBH423), which are dangerouseasy to explode in the presence of water and oxygen. Moreover, both ion intercalation/exchange LPE methods have proven only effective for a specific type of layered materials.17 Although great breakthroughs have been achieved in LPE, several essential characteristics are required: (1) useful for numerous 2DMs, (2) short processing time, (3) chemical stability, (4) ultrathin thickness and relatively large lateral size of nanosheets, and (5) high yield. So far, few reported methods are satisfactory for the above requirements. Herein we propose a microwave-assisted LPE (MALPE) method for the rapid and efficient exfoliation of layered materials into mono- and few-layer nanosheets. We demonstrated this methodology on several 2DMs, including h-BN, gC3N4, BP, TMDs (MoS2, WS2, MoSe2), Zn2(bim)4, and Received: May 11, 2018 Revised: August 9, 2018 Published: August 10, 2018 5932

DOI: 10.1021/acs.chemmater.8b01976 Chem. Mater. 2018, 30, 5932−5940

Article

Chemistry of Materials

Figure 1. Schematic diagram for MALPE. (a) Structural formula of the fluid media. (b) Bulk 2DMs along with fluid media was contained in the quartz vessel, which was further put into a single-mode microwave synthesis system (Discover SP from CEM Corp.). (c, d) Schematic diagrams of the MALPE process with (c) organic solvents and (d) ILs as fluid media.

Ti3C2Tx. Three organic solvents and two PF6−-containing ionic liquids (ILs) were selected as fluid media. Bulk 2DMs were exfoliated in only 30−60 min with a yield up to ∼50%, and nanosheets as large as several micrometers (comparable to the bulk phases) with a uniform thickness frequently 30 min or with a power >30 W, solidification occurred.30,31 When optimizing the microwave parameters, we found high temperature if with low microwave power led to very poor exfoliation. Hence, it is the so-called nonthermal microwave effect28 rather than thermal microwave effect that works on exfoliation. In this procedure, 2DM sheets were thermally agitated, and the intercalation step took place initially at the sheets edges and grain boundaries, gradually leading to exfoliation (Figure 1c,d). For organic solvents, under an alternating electromagnetic field, molecular dipoles oscillated back and forth (i.e., dipole rotation),28 which was helpful in opening the edges of sheets. With the increase of the interlayer distance, it was beneficial for more solvent molecules to intercalate into the interlayers of sheets, gradually leading to a structural disordering and exfoliation (Figure 1c). For ILs, they have been reported as highly effective for the microwave exfoliation of carbon nanotubes30 and graphene.31 In addition to the ionic conduction caused by IL molecules, for PF6−-containing ILs, hydrofluoric acid (HF) species generated by the partial decomposition of ILs under microwave heating prove crucial for MALPE.31 The sterically less-demanding HF species first intercalated randomly into interlayers, which presumably

RESULTS AND DISCUSSION

Solvent Selection and MALPE Process. Three organic solvents, 1-cyclohexyl-2-pyrrolidinone (CHP), 1-methyl-2pyrrolidinone (NMP), and N,N-dimethylformamide (DMF), and two ILs, 1-butyl-3-methylimidazolium hexafluorophosphate (BMIMPF6) and 1-hexyl-3-methylimidazolium hexafluorophosphate (HMIMPF6), were selected as fluid media (Figure 1a), based on the systematic solvent analysis. A desirable MALPE solvent should meet two essential requirements: (1) disperse the materials for a reasonable duration/concentration (Supplementary Note 1) and (2) efficiently absorb the electromagnetic energy. For requirement 1, solvents are demanded with a desired surface tension (∼40 mN m−1; Figure S1 and Table S1) so that the surface energy of 2DMs and solvents are close to each other to minimize the enthalpy of mixing (i.e., high affinity), thus facilitating exfoliation and avoiding reaggregation.18 In addition, it must be noted that both Zn2(bim)4 and Ti3C2Tx are groupterminated with benzimidazole (bim) ligands, hydroxyls, or fluorines and thus are endowed with strong polarity, allowing them to mix well with the polar solvents mentioned above. For requirement 2, with dielectric constants ranging from 25 to 38.25 (Table S1), CHP, NMP, and DMF are polar organic 5933


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Figure 2. Optical images of the nanosheet dispersions and XRD characterizations. (a) Photographs of dilute 2DM ethanol dispersions with a red laser beam passing through. From left to right: pure ethanol, ethanol-dispersed nanosheets exfoliated with DMF, NMP, CHP, BMIMPF6, CHP, HMIMPF6, NMP, and CHP, respectively. (b, c) XRD patterns of bulk and exfoliated (b) h-BN and (c) MoS2 with different solvents. Insets in (b, c): the enlargement at the (002) peak positions. (d) XRD patterns of Ti3AlC2 before etching and exfoliated Ti3C2Tx with CHP.

caused the 2DM interlayer distance expansion, thus further facilitating the intercalation of larger IL molecules and exfoliation (Figure 1d). Optical and XRD Characterizations. After microwave irradiation, the initial unstable 2DM suspensions became homogeneous with negligible sedimentation even after 24 h (Figure S2). The obtained nanosheets were rather stably ethanol-dispersed for up to three months with concentrations of >0.2 mg mL−1. For example, concentrations reached 0.6, 0.4, 0.3, 0.25, and 0.2 mg mL−1 for exfoliated h-BN (with DMF), BP (with HMIMPF6), MoS2 (with CHP), Zn2(bim)4 (with NMP), and g-C3N4 (with BMIMPF6), respectively. Moreover, the obvious Tyndall effect of the dilute dispersions suggested the efficient exfoliation of the 2DMs (Figure 2a). Xray diffraction (XRD) was done on bulk and freshly exfoliated 2DMs (Figure 2b−d and Figure S3). No extra peaks were observed after microwave irradiation, tentatively demonstrating the good purity of the nanosheets. Importantly, the intensities of the (002) peaks originating from the well-organized monolayer stacking significantly decreased by 56.5%−90.9% in different fluid media, clearly demonstrating z-orientation disordering and high-efficiency exfoliation.31,32 Moreover, the (002) peak generally downshifted slightly to a low-degree position, corresponding to an increase in the interlayer distance.32 The intensity decrease and the position shift of the (002) peaks together indirectly verify the occurrence of exfoliation (Figure 2b,c and Figure S3a−c). In general, organic solvents led to more obvious (002) peak intensity decreases and position shifts, thus more effective exfoliation. For Ti3C2Tx as a special case, after etching of Ti3AlC2, multilayer Ti3C2Tx was obtained (Figure S4). After exfoliation, the intensest peaks of Ti3AlC2 at 2θ ≈ 39° disappeared, and peaks from 20° to 40° were still available but very weak (Figure 2d), suggesting effective exfoliation.33 Morphology Characterizations. Transmission electron microscopy (TEM) images show highly electron-transparent nanosheets exfoliated with different fluid media (Figure 3a,c,e−h; Figures S5 and S6), which are in stark contrast to their bulk phases (Figure S7). Layer edges indicated by the yellow arrows at the sheet periphery suggest mono- and fewlayer nanosheets (Figure 3b,d and Figure S8a) with highresolution TEM (HRTEM). HRTEM images along with the selected area electron diffraction (SAED) patterns indicate intrinsic 2DM crystallinity with few defects, even for unstable

Figure 3. TEM characterization of the MALPE nanosheets. (a, c, e− h) TEM images of exfoliated (a) h-BN (with DMF), (c) MoS2 (with CHP), (e) BP (with HMIMPF6), (f) g-C3N4 (with BMIMPF6), (g) Ti3C2Tx (with CHP), and (h) Zn2(bim)4 (with NMP). Insets in (a, c): HRTEM images (balls indicate atoms) and SAED patterns of the red square-marked areas. (b, d) HRTEM images of (b) h-BN and (d) MoS2 showing layer edges of the red square-marked areas in (a, c). Scale bars: 200 nm in TEM images, 1 nm for HRTEM with balls, 2 nm for HRTEM with arrows, and 5 1/nm for SAED patterns.

BP (Figure S8). These results reveal that microwave irradiation results in successfully exfoliated 2DMs of intrinsic structures with little damage and few defects, which are consistent with the XRD results above and will be further confirmed with spectral analysis below. Atomic force microscopy (AFM) further reveals the thicknesses and lateral sizes of the exfoliated 2DMs. For all the 2DMs, the as-obtained nanosheets possess a uniform thickness of approximately 3−4 nm (Figure 4a−f and Figures S9−S11). When determining the layer numbers, the monolayer thickness is better to be overestimated by 1−3 Å due to residual solvents and other effects, such as capillary forces and adhesion.17 We determine approximately 2 layers of h-BN (Figure 4a; considering the particular stack condition, Supplementary Note 2), 2−4 layers of BP (Figure 4b), 3−4 layers of MoS2 (Figure 4c, upper), monolayer of WS2 (Figure 4c, lower), and 3−5 layers of g-C3N4 (Figure 4d) exfoliated with organic solvents or ILs, according to previous values.32,34−37 Notably, despite the presence of strong linkages between the functional groups of Zn2(bim)438 and Ti3C2Tx,27 they can still be exfoliated into ultrathin nanosheets, e.g., ∼3 5934


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Figure 4. AFM analysis of the MALPE nanosheets. (a−f) AFM images of the exfoliated (a) h-BN with (upper) DMF and (lower) HMIMPF6, (b) BP with (upper) HMIMPF6 and (lower) NMP, (c) (upper) MoS2 with CHP and (lower) WS2 with NMP, (d) g-C3N4 with BMIMPF6, (e) Zn2(bim)4 with NMP, and (f) Ti3C2Tx with CHP. (Insets in a−f) White height profiles along the red lines. (g) Thickness histograms of exfoliated BP using HMIMPF6 and NMP. (h) Size histograms of the bulk and exfoliated BP using HMIMPF6 and NMP, respectively. (i) Yield histograms of 2DM nanosheets exfoliated with different solvents. Scale bars: 2 μm.

reaction across the entire volume can be homogenized in an inverted temperature gradient,40 which has been demonstrated as a specific effect of the microwave method. In addition, we found that the as-obtained nanosheets were readily size- and thickness-selected by controlled centrifugation (Figure S14). Exfoliation Efficiency. The nanosheet yields using different solvents are summarized in Figure 4i. The MALPE process here allows for relatively high-yield exfoliation of most 2DMs using both ILs (approximately 20%−25%) and organic solvents (approximately 30%−40%). Remarkably, a yield as high as ∼50% was obtained for h-BN (with DMF). In contrast, with strong linkages between interlayers, both Zn2(bim)4 and Ti3C2Tx had relatively low nanosheet yields with organic solvents, while ILs led to poorer exfoliation. In terms of solvents, organic solvents generally achieved higher yields than ILs, likely originating from the longer-lasting strong molecular dipole rotation of microwave-durable organic solvents under prolonged higher-power irradiation as well as sterically lessdemanding organic solvent molecules facilitating intercalation and exfoliation. By contrast, ILs are more viscous and less microwave-durable and with sterically more-demanding molecules.28 Last but not least, we also found pretreatment parameters including wet-grinding time and the initial powder concentration play a role in the yield when optimizing experiments. After an overall consideration of yield and nanosheet quality (thickness and lateral size), we determined the wet-grinding time as 10 min and selected an initial powder concentration as 10 mg/3 mL (Supplementary Note 3, Figure S15). Spectra Verification and Chemical Stability Analysis. Raman and photoluminescence (PL) spectra were obtained to further confirm the exfoliation. The Raman spectra of BP (Figure 5a) show characteristic peaks at approximately 363, 440, and 467 cm−1.41 Differently, the spectra of BP nanosheets show slight blue-shifts (as large as 2.10 cm−1 (Ag2 mode)), ascribed to the ultrathin thickness of the nanosheets. This is because the oscillation of the P atoms within the monolayer is strengthened once bulk BP is cleaved into individual nanosheets.42 It is worth noting that the spectra of BP

layers for Zn2(bim)4 (Figure 4e and Figure S11a) and 2−3 layers for Ti3C2Tx (Figure 4f and Figure S11b) exfoliated with organic solvents. Generally, about 5%−10% of monolayers were found for most of the 2DMs, while ∼20% of monolayers were found for h-BN, owing to the small original lateral size and the weaker interlayer bonding like graphite. Just like the most widely used sonication-assisted LPE method, the yield of monolayers is relatively low, which proves to be a main challenge for LPE.17 Considering the short processing time, the sub-5 nm thicknesses of these two materials were satisfactory and comparable to previous reports.33,39 Besides, MALPE can also be effectively applied for exfoliation of graphite into ultrathin (∼20% of monolayers) and large lateral-size (generally >5 μm) graphene with a yield up to ∼40% (with DMF, Figure S12). By comparison, Matsumoto and co-workers have realized much higher throughput exfoliation for graphene monolayers with the assistance of molecularly engineered ILs.31 They considered the possible multivalent mode of interaction between special IL molecules and the graphene surface is very important for graphite exfoliation, while the synthesis of the molecularly engineered ILs is rather complex. Thickness and Size Distributions. Furthermore, after statistically analyzing >100 arbitrary bulk sheets and nanosheets (Supplementary Note 2), the major thickness and size distributions of the 2DMs before/after exfoliation are given in Table S3. Taking BP as an example, both ILs (e.g., HMIMPF6) and organic solvents (e.g., NMP) led to effective exfoliation, but little thickness and size difference existed among the solvents (Figure 4g,h). Moreover, one remarkable result showed that the exfoliated nanosheets have lateral sizes that essentially match those of the bulk phases. For example, BP nanosheets have lateral sizes (2−7 μm) comparable to those of the bulk powder (2−8 μm) (Figure 4h). A mere 10%−20% size reduction occurred after exfoliation for almost all the 2DMs (Figure S13d−f and Table S3), while for other widely used exfoliation methods, the value of size reduction is frequently >50%17,20,32 (Table S4). This might be because the microwave heat equilibrium can be reached quickly, and the 5935


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deconvolution of the Mo 3d spectrum. For organic solvents, recent molecular scale computational studies21 have shown that NMP, CHP, and other organic solvent molecules could form tightly packed solvation shells on the nanosheet surfaces (Figure 1c) to facilitate exfoliation, prevent aggregation, and significantly slow the degradation process. For ILs, the imidazolium cations were first linked to the negatively charged 2DM surfaces via attractive Coulombic forces and π−π stacking interactions.46 Then, an IL double-layer structure (see Figure 1d) formed by attracting PF6− species, effectively screening the electrostatic interactions between the dispersed nanosheets, as well as forming a protection solvation shell for nanosheets31,47 to accelerate exfoliation, stabilizes nanosheets and acts as a barrier of oxidative species. Theoretical Simulations. Experimental and COMSOL simulations were further conducted to explore the effects of irradiation factors (time and power) and solvents on the exfoliation effects. h-BN was selected as a typical material for the study. As expected, a shorter irradiation time and a lower irradiation power resulted in lower exfoliation yields (Figure S18), while a yield plateau appeared if irradiated for longer than 60 min with organic solvents. It is due to that a concentration limitation exists in the nanosheet dispersions created with LPE methods,18−21 which is mainly affected by the binding energy balance in 2DMs/solvents dispersions.17,18 Notably, although CHP and DMF have surface tensions close to each other, their ability to exfoliate h-BN differs (DMF > DMF/CHP mixture > CHP, Figure 6a). We attribute the obvious difference to their dielectric properties. The dielectric constant ε′ of DMF is 38.25, while that of CHP is only 25 (Table S1). This parameter directly indicates materials’ ability to store electromagnetic energy via polarization.28 DMF with a higher ε′ can interact more intensely with microwaves, leading to a more obvious weakening of the applied electric field and thus more active sheet−solvent interactions.48 This hypothesis was theoretically validated with electromagnetic simulations. We first built a simplified macro model of the reaction cavity (Figure S19), so the electric field distributions for different solvents can be obtained. Then, a micro model was also built (Figure 6b) for the electric field distribution and the power density distribution in the sheet−

Figure 5. Raman and XPS analysis of the MALPE nanosheets. (a) Raman spectra BP before and after exfoliation with BMIMPF6 and NMP. (b) XPS spectra of exfoliated BP with NMP.

exfoliated with organic solvents (e.g., NMP) show more obvious blue-shifts, indicating smaller average thicknesses of nanosheets. In addition to the blue-shifts of Raman peak positions, intensity decrease and the broadening of Raman peaks together suggest the successful exfoliation of BP.42 Similarly, the exfoliation of WS2 and h-BN is also confirmed (Figure S16). The photoluminescence originating from fewlayer MoS2 is also shown in Figure S17a. MoS2 is known to change the fluorescence properties after exfoliation, which originates from direct band gap PL of MoS2 nanosheets.43 Two broad connected peaks at approximately 632 and 664.2 nm can be observed with 510 nm excitation wavelength, further demonstrating the successful exfoliation of MoS2.44 BP is easy to oxidize in coexistence of vapor, oxygen, and light.34 Meanwhile, TMDs (e.g., MoS2 and WS2) can be oxidized if heated in air or oxygen.45 Thus, Raman and X-ray photoelectron spectroscopy (XPS) were applied to chemical stability confirmation on nanosheets. No oxidation signatures, such as P−O and W−O vibrations, were observed from the Raman spectra of BP (Figure 5a) and WS2 (Figure S16a) nanosheets, indicating the good chemical stability of the nanosheets using either ILs or organic solvents. Moreover, with XPS, the P 2p core-level spectra of BP nanosheets (exfoliated with NMP, Figure 5b) exhibit a P 2p3/2 (130.05 eV) and P 2p1/2 (130.85 eV) doublet of well-crystallined BP. PxOy species with broad peaks in the region of 131.6−136.0 eV are rarely observed.34 Negligible oxidation was also identified in MoS2 nanosheets (exfoliated with BMIMPF6, Figure S17b) after the

Figure 6. Theoretical simulations of factors affecting exfoliation efficiency. (a) h-BN nanosheet yield profile with CHP, DMF, or their mixtures as solvents. (b) A simplified micro model that is a finite element containing an individual bulk h-BN sheet (typically ϕ 500 × 50 nm2) surrounded by the organic solvent. (c) Electric field distributions of organic solvents (left: CHP; middle: VolCHP:VolDMF = 1:1; right: DMF) in macro models. √E: the square root of the electric field strength. (d) Power density distributions of micro models with h-BN sheets surrounded by (left) CHP and (right) DMF. Microwave power set: 140 W. 5936


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high yield (up to 50% of yield). Essential requirements for the MALPE solvents were also proposed through experimental and theoretical simulations. Apart from the desired surface tension, the dielectric constant is a key factor determining the exfoliation effects. Generally, the preferred fluid media with a desired surface tension should also be strongly polar (e.g., with a high dielectric constant for organic solvnets). The findings will guide the seeking for new matching solvents, indicating that the technique would be suitable for the exfoliation of a large variety of 2DMs. The research promotes the development of large-scale synthesis of 2DMs with larger reaction vessels or multiple microwave reactors. Finally, we demonstrate 3D printing of predesigned architectures and patterns, which is essential for many practical applications, such as energy conversion and storage, thermal management, seawater desalination, and so on.

solvent interaction condition. Basic data for the simulations are listed in Table S5. First, compared to CHP, DMF with a higher dielectric constant can store more electromagnetic energy through polarization, leading to more weakening of the applied electric field (Figure 6c). Then, for the sheet−solvent interaction condition, the electric field distribution of the hBN−CHP and h-BN−DMF interaction condition seems similar (Figure S20) because the electric field distribution at the boundary is codetermined by the dielectric properties of both h-BN and solvents. h-BN is a perfect dielectric with a dielectric constant far lower than that of solvents,49 which causes little weakening of the applied electric field on the boundary using either CHP or DMF. Furthermore, the power density (Pd) was calculated using Pd = 2πfε0ε″|E|2, where f is the electromagnetic frequency, ε0 the permittivity of free space (8.85 × 10−12 F/m), ε″ the dielectric loss factor, and E the electric field strength (V/m).48 An intenser power density distribution at the edge sites exists in the more active h-BN DMF interaction (Figure 6d) . Hence, on the basis of experimental and simulation results, we thought the so-called nonthermal microwave effect28,40 in MALPE can be enhanced through effective electromagnetic energy absorption of polar solvents by using polar solvents (e.g., organic solvents with a high dielectric constant), thus leading to more effective exfoliation. 3D Printing. The direct ink writing (DIW) technique has realized the programmable production of a range of materials and architectures.50−54 We performed DIW with exfoliated 2DMs as the primary building blocks of the inks (Figure S21a). With the addition of small amount GO, by virtue of its strong electrostatic interactions with 2DM sheets, the concentrated hBN-based ink (∼40 mg mL−1 ) showed a prominent viscoelastic response with obvious shear-thinning behavior (Figure S22a), and the values of the storage modulus (G′) and loss modulus (G″) reached ∼2 × 104 Pa and ∼4 × 103 Pa, respectively, with a yield stress of ∼103 Pa (Figure S22b). All the properties are essential for patterning 3D macroscopic architectures with 2DM-based inks. A programmable “butterfly” pattern was successfully printed with the exfoliated h-BN nanosheet-based ink (Figure S21b). We also printed a microlattice structure with MoS2-based ink (Figure S21c), and the components were uniformly distributed (Figure S23). A similar approach can be extended to other 2D nanosheetbased systems for the construction of hybrid architectures toward various potential applications.

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EXPERIMENTAL SECTION

Materials. Bulk h-BN (99.9%), MoS2 (99.9%), WS2 (99.9%), and MoSe2 (99.9%) powders, lithium fluoride (LiF, 99%), 1-butyl-3methylimidazolium hexafluorophosphate (BMIMPF6, 97%), 1-hexyl3-methylimidazolium hexafluorophosphate (HMIMPF6, 97%), 1cyclohexyl-2-pyrrolidinone (CHP, 99%), 1-methyl-2-pyrrolidinone (NMP, >99.0%), and dimethyl sulfoxide (DMSO, anhydrous solvent grade) were all purchased from Aladdin Industrial Corporation. Bulk black phosphorus (BP, >99.998%) powder was purchased from XFNANO Materials Tech Co., Ltd. Cyanamide (97.0%) was purchased from Credit Chemical Co., Ltd. The MAX phase titanium aluminum carbide (Ti3AlC2, 98%, 200 mesh) powder was purchased from Forsman Tech Co., Ltd. ZnCl2 (≥98.0%), N,N-dimethylformamide (DMF, ≥99.5%), diethylamine (DEA, ≥98.0%), and ethanol (EtOH, ≥99.7%) were all purchased from Sinopharm Chemical Reagent Co., Ltd. Benzimidazole (bim, 98.0%) was purchased from Energy Chemical Technology Co., Ltd. Raw graphite powder (99.99%) was purchased from Sigma-Aldrich Co., Ltd. Bulk 2DMs Synthesis. Bulk g-C3N4 powder was prepared via sintering cyanamide at 550 °C for 4 h under a continuous Ar flow.55 Multilayer Ti3C2Tx (ML-MXenes phase) powder was synthesized using a selective etching route based on a previously reported method.56 Bulk poly[Zn2(benzimidazole)4] (i.e., Zn2(bim)4) powder was synthesized as described in a previous study41 (see XRD in Figure S3d). MALPE Process. 10 mg of bulk 2DM powder along with 0.2 mL of MALPE solvent (BMIMPF6, HMIMPF6, CHP, NMP; DMF was only tested for graphite, h-BN and MoS2) was first wet-ground for 10 min using a mortar and then transferred into a 10 mL quartz vessel (a matching perfluoroalkoxyalkane polymer (PFA) inner vial was used to protect the quartz reactor from HF corrosion for PF6−-containing ILs) with another 2.8 mL of solvent. The operation of BP and Ti3C2Tx was conducted in an Ar glovebox. During microwave irradiation with a single-mode microwave synthesis system (Discover SP from CEM Corp.), high-speed stirring was used to homogenize the whole reaction. In addition, CHP, DMF, and their mixtures (VolCHP:VolDMF = 1:0, 2:1, 1:1, 1:2, and 0:1) were applied as fluid media to exfoliate hBN to explore the effect of solvents on exfoliation. The effects of irradiation time (15, 30, 45, 60, and 90 min; constant power, 140 W) and power (60, 80, 100, 120, and 140 W; constant time, 60 min) on the exfoliation effects were also investigated using DMF to exfoliate hBN. Post-Treatment Process. The as-obtained suspension was first diluted with 5 mL of DMSO, subsequently rinsed using ethanol, and filtered with a polytetrafluoroethylene (PTFE) filter membrane on a sand core filtration system. The solid residue was redispersed in ethanol by 5 min of ultrasonic agitation and centrifuged with 1500 rpm/20 min (2000 rpm/30 min for h-BN and Zn2(bim)4 due to their small sheet size) to remove the thick sheets or 5000 rpm/20 min for thinner nanosheets. The supernatant was further filtered using a PTFE membrane to gather the solid residue and vacuum-dried at 40

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CONCLUSIONS To summarize, we demonstrated microwave irradiation of eight typical bulk layered materials in specific fluid media, which allowed for rapid and efficient exfoliation of mono- and few-layer nanosheets of excellent chemical stability and large lateral size. The advantages over other LPE methods are the following: (1) Useful for numerous 2DMs. 2DMs including hBN, g-C3N4, BP, TMDs (MoS2, WS2, MoSe2), Zn2(bim)4, and Ti3C2Tx were tested for successful exfoliation. Notably, beyond the well-studied layered materials, some new emerging materials such as MOFs and MXenes can also be effectively exfoliated. (2) Short processing time (30−60 min). (3) Chemical stability. The 2DMs maintain their intrinsic structures with negligible defects (e.g., oxidation), even for easily oxidized 2DMs (e.g., BP and MoS2). (4) Relatively large lateral size of nanosheets as compared to other LPE methods. (5) Ultrathin thickness (mainly three layers) and relatively 5937


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Chemistry of Materials °C for 20 h for further investigations. In addition, the nanosheet yield was back-calculated by weighing the sediment after vacuum-drying on aluminum foil. The same operation was repeated for 5 times for the same exfoliation conditions, and the average was taken for a more reliable calculation of the corresponding yield. Characterization. Powder XRD was conducted with a Rigaku Dmax/2550 diffractometer. Morphology characterizations were performed using SEM (Hitachi S-4800), TEM (JEM-2100F operated at 200 kV, but 300 kV for HRTEM), and AFM (Bruker Mulimode 8). Raman spectroscopy was conducted using a Thermo Scientific DXR2xi (532 nm excitation laser). PL spectroscopy was conducted with a luminescence spectrometer (PerkinElmer LS 55, 510 nm light excitation). XPS spectra were obtained using the instrument (AXIS UltraDLD). Rheological characteristics of the 3D printing inks were obtained using a rheometer (HAKKE MARS3). Electromagnetic COMSOL Simulations. Electromagnetic simulations were performed using the frequency-domain analysis of the electromagnetic wave in the radio-frequency (RF) module in COMSOL multiphysics 5.2. First, we simulated the electric field distribution in two different organic solvents (CHP and DMF) and their mixture (VolCHP:VolDMF = 1:1) using a macro model. Second, we further simulated the electric field and the power density distribution in h-BN solvent (CHP, DMF, or merely with air) interaction condition using a micro model. Basic data including the model dimensions, the simulation parameters, formulas, and the required physical parameters of each element are listed in Table S5. 3D Printing. Single-layered GO sheets were synthesized using a modified Hummers’ method. The 2DM-based inks were prepared with a solution mixing method via a solvent evaporation process until a gel-like mixture was formed. In a typical procedure, the GO aqueous dispersion (40 mL, 2.3 mg mL−1) was dropwise added into an aqueous dispersion of the exfoliated h-BN (180 mL, 2 mg mL−1) or MoS2 (90 mL, 1 mg mL−1) under constant stirring. Next, the suspension was bath sonicated (KQ-300B, operating at 300 W for sonication) for 10 min to facilitate the dispersion of each components. After that, the suspension was kept at 85 °C for ca. 10 h to facilitate solvent evaporation. The obtained soft-gel inks were squeezed into a 30 mL syringe, which was attached with borosilicate micronozzles for 3D printing. The 3D patterns were printed with “Bioprinter” (Shining3d Co., Ltd.) and freeze-dried for further characterizations.

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*E-mail: [email protected] (T.X.F.). ORCID

Jingsan Xu: 0000-0003-1172-3864 Tongxiang Fan: 0000-0003-4255-5138 Notes

The authors declare no competing financial interest.

ACKNOWLEDGMENTS

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REFERENCES

We greatly acknowledge the financial support by the Foundation for National Natural Science Foundation of China (51425203 and 51772191), Natural Science Foundation of Shanghai (17ZR1441100), and Joint fund of the equipment pre Research Ministry of Education (6141A02033605).

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.chemmater.8b01976. Supplementary Notes: supplementary for the solvent analysis; size and thickness statistics with AFM; effects of pretreatment parameters on the yield. Supplementary figures and tables: surface tensions/dielectric constants/ boiling points and optimized reaction conditions of different solvents; optical images of 2DM dispersions; XRD/Raman/PL/XPS patterns, SEM/TEM/AFM images, thickness/size distributions of 2DM sheets; comparison of size reductions of nanosheets exfoliated with different methods; nanosheet yield profiles against pretreatment parameters and microwave irradiation factors; basic data used in the COMSOL simulations; COMSOL simulation macro model and results of electric field distributions of micro models; schematic illustration, optical/SEM images, rheological properties, and element mapping results of 3D printing (PDF)

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected] (H.Z.). 5938


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Two-Dimensional Nanosheets by Rapid and Efficient Microwave

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