Large-Scale, Highly Efficient, and Green Liquid-Exfoliation of Black

Letter www.acsami.org

Large-Scale, Highly Efficient, and Green Liquid-Exfoliation of Black Phosphorus in Ionic Liquids Wancheng Zhao,† Zhimin Xue,‡ Jinfang Wang,† Jingyun Jiang,† Xinhui Zhao,† and Tiancheng Mu*,† †

Department of Chemistry, Renmin University of China, Beijing 100872, China Beijing Key Laboratory of Lignocellulosic Chemistry, College of Materials Science and Technology, Beijing Forestry University, Beijing 100083, China

‡

S Supporting Information *

ABSTRACT: We developed a facile, large-scale, and environmentally friendly liquid-exfoliation method to produce stable and high-concentration dispersions of mono- to few-layer black phosphorus (BP) nanosheets from bulk BP using nine ionic liquids. The prepared suspensions can stabilize without any obvious sedimentation and aggregation in ambient air for one month. In particular, the concentration (up to 0.95 mg mL−1) of BP nanoflakes obtained in 1-hydroxyethyl-3methylimidazolium trifluoromethansulfonate ([HOEMIM][TfO]) is the highest reported for BP nanosheets dispersions. This work provides new opportunities for preparing atomically thin BP nanosheets in green, large-scale, and highly concentrated processes and achieving its in situ application. KEYWORDS: black phosphorus, ionic liquids, liquid-exfoliation, phosphorene, high concentration

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wo-dimensional (2D) layered materials1 represented by graphene2 have emerged as a very attractive class of nanomaterials because of their unique structural and electronic properties. In addition to layered graphene, there are other attractive 2D monolayer or few-layer materials such as layered transition metal dichalcogenides (MoS2, MoSe2, MoTe2, WS2, WTe2, TiSe2, TaSe2, NbSe2, NiTe2)3 and boron nitride (BN)4 nanosheets. Black phosphorus (BP)5−7 (Figure 1a and Figure 1b), a 2D layered semiconductor, has received a great deal of attention over the last two years. Importantly, it has enormous potential to be used in high-performance optoelectronic and electric devices because of its high carrier mobility,6 large on/off ratio,8 and thickness-dependent band gap5,8 ranging from 0.3 eV for

bulk to 2.0 eV for monolayer (termed phosphorene). All of the aforementioned fascinating properties make 2D BP an ideal competitor for the applications in field-effect transistors,5 gas sensors,9 photodetectors,8 photothermal agents,10 and so forth. Particularly, compared to the bulk BP, few-layer BP is more suited for applications in near- and mid-infrared optoelectronic devices because of its direct band gap and higher specific surface area. Thus, we need to find an efficient method to exfoliate bulk BP into atomically thin BP nanosheets. Though micromechanical exfoliation method has been used to produce BP atomic layers by breaking down the strong interlayer interactions,6 it is still a small-scale method that is not suitable for mass production or industrial applications. Inspired by liquid-phase exfoliation technique applied to other 2D materials,11 several efforts have been made to get large-scale and solution-processable single- and few-layer BP nanoflakes.10,12−17 Brent et al. first reported the liquid exfoliation of BP in N-methyl-2-pyrrolidone (NMP) to produce few-layer phosphorene,12 and following researchers kept trying to prepare large-scale, stable, highly concentrated solution dispersions of BP atomic layers via exfoliating in conventional organic solvents,13,16 combining probe sonication and bath sonication,10 grinding17 and shearing.15 However, these liquidphase exfoliation processes are generally prepared in environmentally unfriendly organic solvents. Moreover, the prepared suspensions of BP nanosheets are still at relatively low

Figure 1. (a) Structure of BP crystal. (b) Top view of a. (c) Photograph of BP dispersions in nine different ILs after 4000 rpm centrifugation. (d) Schematic of green IL-exfoliation of bulk BP into BP nanosheets. © XXXX American Chemical Society

Received: November 7, 2015 Accepted: December 8, 2015

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DOI: 10.1021/acsami.5b10734 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

Letter

ACS Applied Materials & Interfaces

Figure 2. Optical characterization of IL-exfoliated BP nanosheets. (a) Photograph of the atomically thin BP dispersions in [BMIM][TfO] and [HOEMIM][TfO] (left) and the Tyndall effect of diluted dispersions (right). (b) Raman spectra of bulk BP and IL-exfoliated BP nanosheets in [BMIM][TfO] and [HOEMIM][TfO]. (c) Standard curves of absorbance per length of the cell (A/l) at different concentrations of BP for λ = 1188 nm in [BMIM][TfO] and [HOEMIM][TfO]. (d) Histograms of IL-exfoliated BP nanoflakes in [BMIM][TfO] and [HOEMIM][TfO] describing their size distribution.

an ice bath for 24 h to prepare suspensions of BP nanoflakes. The resulting dispersions were centrifuged at 4000 rpm for 45 min to remove unexfoliated BP crystals and the stable supernatants containing atomically thin BP nanosheets were carefully collected and retained for use. (see more process details in the Supporting Information). Nine different ILs (see chemical structures in Figure S1) were used to successfully achieve this approach including 1-butyl-3-methylimidazolium tetrafluoroborate ([BMIM][BF4]), [BMIM] trifluoromethansulfonate ([TfO]), [BMIM] bis((trifluoromethyl)sulfonyl)imide ([Tf2N]), 1-Ethyl-3-methylimidazolium ([EMIM]) [Tf 2 N], [EMIM][BF 4 ], 1-hexyl-3-methylimidazolium ([HMIM]) [BF4], 1-octyl-3-methylimidazolium ([OMIM]) [BF4], 1-hydroxyethyl-3-methylimidazolium ([HOEMIM]) [TfO], and [HOEMIM] [BF4]. We have chosen dispersions obtained in [BMIM][TfO] (a common IL) and [HOEMIM][TfO] (a functionalized IL) as representatives (left in Figure 2a) to do a series of characterization to prove that bulk BP can be exfoliated into atomically thin BP dispersions in ILs. Such dispersions can stabilize without any obvious sedimentation and aggregation at ambient air for one month, which are verified by the Tyndall effect for diluted suspensions (right in Figure 2a). As shown in Figure 2b, the successful exfoliation of BP into phosphorene was confirmed by Raman spectra. Peaks of BP nanoflakes, exfoliated by [BMIM][TfO] at ∼360, ∼437, ∼466 cm−1 and exfoliated by [HOEMIM][TfO] at ∼363, ∼437, ∼466 cm−1, are attributed to the A1g, B2g, and A2g phonon modes of few-layer BP, indicating that BP atomic layers are crystalline owing to the modes which are consistent with the Raman peaks of BP nanosheets prepared by mechanical exfoliation.27 Compared to the bulk BP, three vibrational modes A1g, B2g, and A2g of ILexfoliated BP nanosheets show slightly blue-shifted, confirming bulk BP was exfoliated into atomically thin BP dispersions in ILs.28 The peak at ∼520 cm−1 meets with TO phonon mode for the silicon substrate.12,13 Figure 2c displays the standard curves of absorbance per length of the cell (A/l) at different

concentrations. Therefore, alternative liquid-phase methods, which are facile, high-concentration, nontoxic, and environmentally friendly, are very desirable. Ionic liquids (ILs),18 a kind of molten salts at room temperature or a bit higher point, are popular green solvents in recent decades. Unlike traditional organic solvents, ILs possess many excellent properties18 such as nonvolatility, high thermal stability,19 high viscosity, high ionic conductivity, nontoxicity, versatile solubility and solvent recyclability. Thus, ILs are widely utilized in catalysis, synthesis, contaminant removal,20 dissolution of biomass,21 and energy-related applications.22 In particular, researchers have reported that ILs could effectively exfoliate 2D materials to yield stable and high-concentration suspensions of nanoflakes including graphenes,23 BN layers24 and MoS2 nanosheets.25 Herein, for the first time, we demonstrate the facile liquidexfoliation of bulk BP into mono- to few-layer BP nanosheets with high efficiency in ILs combining mild grinding and weak sonication. This process produces stable dispersions (Figure 1c) of BP atomic layers with high concentrations (up to 0.95 mg mL−1), and there is not any obvious sedimentation and aggregation at ambient temperature for one month. To the best of our knowledge, the concentration (∼0.95 mg mL−1) is the highest reported for BP nanosheets dispersions. This work provides new opportunities for preparing atomically thin BP nanosheets in green, large-scale, and high-concentration processes. Moreover, the dispersions of 2D BP nanosheets exfoliated in ILs could apply to, without any treatment, in situ fabricate BP field-effect transistor by ionic-liquid gating.26 Briefly, IL-exfoliation involves grinding with ILs followed by ice-bath sonication processes in ILs. As schematically shown in Figure 1d, bulk BP (30 mg) was ground with ILs (0.5 mL) into small pieces for 20 min using an agate mortar with a pestle, which provided mechanical shear forces to significantly reduce exfoliation time as the surface area of the chunk BP increased by orders of magnitude.16,17,25 Then, mixtures were dispersed in ILs (3 mg mL−1 BP in ILs) and mild sonicated (100 W) in B

DOI: 10.1021/acsami.5b10734 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

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ACS Applied Materials & Interfaces concentrations of BP for λ = 1188 nm (1.04 eV, five-layer band gap15) in [BMIM][TfO] and [HOEMIM][TfO], which suggest well-dispersed nanosheets in both supernatants. The extinction coefficients (α) for [BMIM][TfO] and [HOEMIM][TfO] suspensions were calculated, according to the Lambert− Beer law, to be 347.1 and 110.5 mL mg−1 m−1, respectively. Thus, the concentrations of [BMIM][TfO] and [HOEMIM][TfO] dispersions after centrifugation were deduced to be around 0.23 and 0.91 mg mL−1, consistent with the results obtained by weighing (Table 1). Subsequently, the size

(EDX) spectrum imaging (Figure S3) indicates individual BP nanoflakes prepared in [HOEMIM][TfO] are of high purity and crystallinity. Elemental map for phosphorus (Figure 3b) was extracted from the spectrum image, revealing a homogeneous phosphorus distribution and agreeing with the nanosheet location. The thickness distribution is one of the most significant data in measuring exfoliation performance. Atomic force microscopy (AFM) was introduced to investigate the topographic morphology (Figure 3c) of ultrathin BP layers obtained in [HOEMIM][TfO]. The AFM results show that the heights are 3.58, 5.50, and 8.90 nm, verifying that bulk BP was exfoliated into few-layer BP nanoflakes successfully. Distinguishable edges of the exfoliated sheets and overlapping nanoflakes were easily observed from AFM image. The heights of every stacked sheet in ladder-shaped BP nanoflakes were less than 2 nm, corresponding to about two to four individual BP layers.14,17 Morphology characterization of IL-exfoliated BP nanosheets in [HOEMIM][TfO] is shown in Figures S2−S4, which suggests BP layers obtained in [BMIM][TfO] were also natural, pure, and homogeneous phosphorus distribution. Transmission electron microscopy (TEM) was performed to further characterize exfoliated BP sheets using [HOEMIM][TfO]. Figure 4a shows a typical low-magnification TEM

Table 1. Properties of Exfoliated BP Nanosheets in Various ILs IL

conc. (mg mL−1)a

yield (%)c

surface tension (mJ m−2)d

[BMIM][BF4] [BMIM][TfO] [BMIM][Tf2N] [EMIM][Tf2N] [EMIM][BF4] [HMIM][BF4] [OMIM][BF4] [HOEMIM][TfO] [HOEMIM][BF4] NMP

0.29 0.22 0.17 0.10 0.75 0.73 0.14 0.95 0.91 0.40b

9.7 7.3 5.7 3.3 25.0 24.3 4.7 31.6 30.3

45e 35e 33e 36e 53e 37e 33e 56f 65e 40e

a

Average concentration of BP dispersion. bData taken from ref 13. Average yield of BP suspension. dSurface tension of IL at 298 K. e Data taken from ref 29 and references therein. fMeasured by the maximum bubble pressure method at 298 K. c

distributions (Figure 2d) of IL-exfoliated BP nanoflakes were provided by dynamic light scattering (DLS) spectroscopy and the average lateral size for [BMIM][TfO] and [HOEMIM][TfO] solvents were about 479 and 459 nm. Figure 3a is scanning electron microscopy (SEM) of ILexfoliated BP nanosheets in [HOEMIM][TfO], showing the morphology of the synthesized BP nanoflakes and lateral dimensions of hundreds of nanometers. Energy dispersive X-ray Figure 4. Electron microscopy characterization of exfoliated BP sheets using [HOEMIM][TfO]. (a) Typical low-magnification TEM image. The arrows point to the obvious wrinkles and distinguishable edges. (b) SAED pattern. (c) HRTEM image. (d) Magnified HRTEM image taken from a selective area in c.

image, revealing ultrathin 2D nanoflakes with obvious wrinkles (pointed by the arrow on the right) and distinguishable edges (pointed by the arrow on the left). The lateral dimensions of BP flakes were hundreds of nanometers, consistent with the SEM image. We employed the selected area electron diffraction (SAED) pattern (Figure 4b) to confirm that exfoliated product was of high-quality single-crystal structure with orthorhombic crystalline character, in good agreement with previous reports 13,14 of liquid-exfoliation. High-resolution TEM (HRTEM) image (Figure 4c) corroborates the result, suggesting that BP flakes obtained from [HOEMIM][TfO] are atomic-scale uniformity. 4× magnified HRTEM image taken from a selective area in ) shows the atomic structure of IL-exfoliated BP sheets, as seen in Figure 4d. Lattice fringes of 0.28, 0.34, and 0.42 nm were observed, matching the monolayer BP structure.16 Moreover, lattice parameters acquired by HRTEM verified that IL-exfoliated BP flakes

Figure 3. Morphology characterization of IL-exfoliated BP nanosheets in [HOEMIM][TfO]. (a) SEM image. (b) EDX elemental map for phosphorus. (c) AFM image and corresponding height profiles along the drawn lines. C

DOI: 10.1021/acsami.5b10734 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

Letter

ACS Applied Materials & Interfaces maintained the unique crystalline nature. Related electron microscopy characterization (Figures S5 and S6) of BP flakes obtained in [BMIM][TfO] confirmed the above conclusion again. Concentrations and yields of IL-exfoliated BP dispersions are given in Table 1. All ILs can efficiently exfoliate bulk BP into highly concentrated BP nanosheets dispersions. Coulombic force and π−π interactions between aromatic ILs cations and phosphorene layers play crucial roles in the exfoliation of BP sheets and stabilization of exfoliated BP flakes.23 Specifically, in Table 1, the concentration (0.95 mg mL−1) of BP nanoflakes obtained in [HOEMIM][TfO] is much higher than the concentration (0.4 mg mL−1)13 obtained in NMP, which is highest in previous reports. From Table 1 and Figure S7, ILs with larger surface tensions generally give higher concentration dispersions, because larger surface tensions29 are beneficial to break down interlayer van der Waals forces of bulk BP and prevent the detached BP layers from restacking.13 A clear anion effect on the concentrations was observed with the order: [BF4] > [TfO]> [Tf2N]. This trend may be attributed to the order of surface tension and hydrogen-bond ability. The comparison of [EMIM][BF4], [BMIM][BF4], [HMIM][BF4], [OMIM][BF4], and [HOEMIM][BF4] indicates that the cations could also strongly affect concentrations via changing cationic chain length. The higher concentrations of BP suspensions can be produced in [HOEMIM][TfO] and [HOEMIM][BF4] perhaps because of more hydrogen bonds introduced by a hydroxyethyl group on the cation. Viscosities of ILs are 1−3 orders of magnitude higher than those of conventional organic solvents. This property may exert a strong effect on the rate of mass transport within solution18 and prevent IL-exfoliated BP nanoflakes from restacking. The viscosity of [HMIM][BF4] (177 cP at 303 K)30 is higher than other ILs (28−90 cP at 303 K),30 it may be the reason that [HMIM][BF4] with relatively low surface tension gives high-concentration suspension (Table 1 and Figure S7). Besides, strongly cohesive dipolar, solvent planarity in solvents can further make ILs appropriate solvents for the preparation of atomically thin BP nanosheets.21,23 In summary, we developed a facile, large-scale, highly concentrated, and environmentally friendly liquid-exfoliation method to produce suspensions of mono- to few-layer BP nanosheets from bulk BP using ILs. The prepared dispersions can stabilize without any obvious sedimentation and aggregation in ambient air for one month. To the best of our knowledge, the concentration (up to 0.95 mg mL−1) of BP nanoflakes obtained in [HOEMIM][TfO] is the highest reported for BP nanosheets dispersions. BP layers exfoliated in ILs were with high purity, crystalline nature, and atomic-scale uniformity ensured by versatile characterization. In addition, the ILs filtrate collected from BP suspensions could be recycled, and the dispersions of 2D BP nanosheets exfoliated in ILs may apply to fabricate BP field-effect transistor by ionic-liquid gating without any treatment.

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sheets in [HOEMIM][TfO], and supplementary figures (PDF)

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors thank National Natural Science Foundation of China (21173267, 21473252) for financial support.

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REFERENCES

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsami.5b10734. Detailed exfoliation procedure instrumentation used, chemical structures of ionic liquids, morphology and electron microscopy images of IL-exfoliated BP nanoD

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DOI: 10.1021/acsami.5b10734 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX