Soluble, Exfoliated Hexagonal Boron Nitride Nanosheets | The

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Soluble, Exfoliated Hexagonal Boron Nitride Nanosheets Yi Lin,*,† Tiffany V. Williams,‡ and John W. Connell‡ National Institute of Aerospace, 100 Exploration Way, Hampton, Virginia 23666-6147 and ‡Mail Stop 226, Advanced Materials and Processing Branch, NASA Langley Research Center, Hampton, Virginia 23681-2199

J. Phys. Chem. Lett. 2010.1:277-283. Downloaded from pubs.acs.org by EAST CAROLINA UNIV on 01/09/19. For personal use only.

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ABSTRACT Hexagonal boron nitride (h-BN), the isoelectric analogue of graphite, was functionalized using lipophilic and hydrophilic amine molecules. The functionalization induced the exfoliation of the layered structure of h-BN, resulting in few-layered and monolayered nanosheets soluble in common organic solvents and/or water. The soluble h-BN nanosheets were characterized using various solution-phase and solid-state techniques. For example, the optical extinction coefficients of the h-BN nanosheets in homogeneous dispersions were estimated to be much lower than those of graphene sheets, confirming their low-colored nature. Solution-phase NMR spectroscopy supported the mechanism that the amino groups of the functional molecules complex to the electron-deficient boron atoms on the h-BN nanosheet surfaces in terms of Lewis acid-base interactions. Results from other microscopic and spectroscopic characterizations of these functionalized two-dimensional nanomaterials are also presented, and the implications of the reported versatile and effective functionalization strategy are discussed. SECTION Nanoparticles and Nanostructures

first monolayered graphene sheets1-3) was successfully applied to the preparation of minute quantities of h-BN nanosheets. More recently, such nanosheets have been produced by sonicating h-BN crystals in a 1,2-dichloroethane solution of a poly(phenylenevinylene) polymer.20 During the preparation of this manuscript, there was also a report on the direct sonication of the h-BN powder in N,N-dimethylformamide (DMF) for nanosheet preparation by taking advantage of the exfoliation capability provided by the polar solvent.21 Here, we present a facile chemical method to isolate monoand few-layered h-BN nanosheets from commercially available powder. Lewis bases such as amine molecules with long lipophilic or hydrophilic chains were used to form complexes or adducts with the electron-deficient boron atoms on h-BN. The complexation of a Lewis base with h-BN facilitated the exfoliation of the layered structure of the bulk material, resulting in thin planar nanosheets that are readily dispersible, or “soluble”, in organic solvents and/or water. The method is potentially scalable for h-BN nanosheet dispersions useful for a variety of applications. The starting h-BN powder was populated with particles with lateral dimensions of a few hundred nm to several μm (see Supporting Information). The thicknesses of most particles were between tens to hundreds of nm. Octadecylamine (ODA, CH3(CH2)17NH2) and an amine-terminated polyethylene glycol [PEG, O,O0 -bis(3-aminopropyl)polyethylene glycol, NH2(CH2)3(OCH2CH2)nO(CH2)3NH2, n ∼ 35] were used as the

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ecently, significant scientific attention has focused on the isolation and properties of single-layer graphene sheets that were previously thought to be insufficiently stable to exist.1-3 These free-standing novel twodimensional materials, often prepared by the exfoliation of bulk graphite samples, have been found to possess excellent quantum transport and mechanical properties that are comparable or superior to one-dimensional carbon nanotubes. As the isoelectric analogue of graphite, hexagonal boron nitride (h-BN) is also naturally comprised of layered structures. The adjacent layers of h-BN are held together by van der Waals forces, with an interlayer spacing of ∼0.33 nm.4 Although h-BN shares similar mechanical strength and thermal conductivity properties with graphite, it has a large band gap (∼4-6 eV), making it an insulator (or a wide-band-gap semiconductor).5 The theoretical investigations on properties of monolayered h-BN nanoribbons (i.e., nanosheets with narrow widths) have just begun to emerge.6-15 For example, it was predicted that, different from their bulk counterparts, h-BN nanoribbons with zigzag edges might exhibit some metallic properties.6,7 Their band structure might also be tunable with either an applied electric field8-11 or planar strain.12 Experimentally, there were only a few reports on the bottom-up16-18 and top-down approaches19-24 to prepare thin h-BN nanosheets. In regard to the latter, where bulk h-BN is used as the starting material, wet oxidation to intercalate and exfoliate the layered structures, one of the common chemical methods for graphene production, has not met with success most likely because of the well-known oxidation resistance of the BN polymorphs.25 Alternatively, the micromechanical cleavage technique19,24 (i.e., the “scotch-tape” method to isolate the

r 2009 American Chemical Society

Received Date: November 2, 2009 Accepted Date: November 19, 2009 Published on Web Date: November 30, 2009

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DOI: 10.1021/jz9002108 |J. Phys. Chem. Lett. 2010, 1, 277–283

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Lewis bases in the functionalization and exfoliation of h-BN. These two amine molecules have previously been extensively studied for the functionalization of carbon nanotubes.26-32 It should also be noted that PEG was the first organic species used to functionalize boron nitride nanotubes by a similar interaction between the amine end and the boron atoms on the nanotube surface.33 Some other Lewis base molecules such as alkylamines and alkylphosphines were also successfully applied on boron nitride nanotubes.34-38 Similar to conditions used for both carbon and boron nitride nanotube functionalizations, the reactions of ODA or PEG with h-BN powder were conducted in the melt by heating the mixture for a prolonged period (4-6 days) under a nitrogen atmosphere. After reaction, the extraction procedure included the addition of an appropriate solvent [tetrahydrofuran (THF) for ODA and deionized water for PEG] to the reaction mixture followed by brief sonication and centrifugation (∼3000 g) to separate the supernatant dispersion from the residue. The extraction was repeated several times, and the supernatants were combined as the soluble fraction that contained ODA- (ODA-BN) or PEG-functionalized h-BN nanosheets (PEG-BN). According to the weight balance, the soluble fraction typically consisted of 1/10 to 1/5 of the starting h-BN weight (e.g., 5-10 mg from 50 mg starting material), a significantly higher yield than previously reported.21 After the solvent was evaporated, the solid product could be redissolved in various solvents, forming homogeneous h-BN nanosheet dispersions with h-BN equivalent solubilities on the order of 0.5-1 mg/mL. For example, PEG-BN was soluble in water as well as some organic solvents such as chloroform. Besides THF, ODA-BN was soluble in chloroform, methylene chloride, and toluene. These dispersions were colorless39 and transparent but with “milky” appearances (somewhat similar to an emulsion). When dilute (e.g., the h-BN equivalent concentration [h-BN] ∼ 0.01 mg/mL), the milkiness became less significant, and the dispersions appeared just as a neat functional molecule solution without the presence of h-BN (Figure 1a). However, different from the neat functional molecule solution, the functionalized h-BN dispersions exhibited the Tyndall effect, similar to what was found in graphene dispersions.40 As shown in Figure 1b, the path of a laser beam could be clearly seen through the dispersion due to the scattering of the h-BN nanosheets. These functionalized h-BN nanosheet dispersions remained quite stable for up to a few months. There was a small amount of white-colored precipitate that appeared over time, which was likely due to the internanosheet aggregation associated with gradual decomplexation. The optical absorption spectra of the functionalized h-BN nanosheet dispersions were featureless in the visible, with low extinction (or high transmission) tailing into the near-IR region. For example, the transmittance of a THF dispersion of ODA-BN at a [h-BN] of ∼0.14 mg/mL (Figure 1c) was over 85% at 400 nm and above. The extinction values obeyed the Lambert-Beer's Law along the wavelengths measured, indicating that the h-BN nanosheets were well-dispersed. The extinction coefficients (ε) for h-BN nanosheets for ODA-BN in THF and PEG-BN in water were similar, with estimated values


Figure 1. Photographs of (a) THF solutions of ODA (left) and ODABN (right) and (b) the same solutions with the irradiation of a laser beam from the left. (c) Optical absorption spectrum and the corresponding transmission spectrum of a THF dispersion of ODA-BN with an approximate h-BN equivalent concentration ([h-BN]) of ∼0.14 mg/mL. The inset shows the Lambert-Beer's Law plots of the sample at 1000 (0), 600 (Δ), and 300 nm (O). [hBN] was estimated from the thermogravimetric analysis data of hBN content in the sample.

of ∼3 L mol-1 cm-1 at 1000 nm and ∼10 L mol-1 cm-1 at 600 nm, orders of magnitude lower than those for graphene sheets.41,42 This confirmed the expectation that the h-BN nanosheets are excellent low-colored/transparent materials in the visible and near-IR. It should be noted that the scattering of the h-BN nanosheets, rather than their absorption, may be dominant in the total extinction in the visible and near-IR regions. Therefore, it may be expected that these ε values should be even lower if additional lateral size reduction of the nanosheet species is achieved. The band gap transition of h-BN could not be directly observed from a THF dispersion of ODA-BN due to the overwhelming solvent interference but could be located at ∼204 nm (∼6.1 eV) when the dispersion was dried as a thin solid layer on a quartz substrate followed by thermal removal of most functionalities (e.g., 450 °C in N2 for 3 h) (see Supporting Information). A similar value was found from a colorless aqueous solution of PEG-BN40 since the cutoff wavelength of water is in the deeper UV (190 nm). These values are consistent with the literature data on bulk h-BN with no functionalities,43,44 suggesting that the functionalization and the exfoliation had little effect on the electronic structure of h-BN. Transmission electron microscopy (TEM) images of the functionalized h-BN samples showed that the h-BN

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Figure 2. Typical low-magnification TEM images of (a) a PEG-BN sample, (b) the supernatant, and (c) the residue from the same sample subjected to further centrifugation at ∼10000 g; (d) a TEM image of a h-BN nanosheet in the same PEG-BN sample and (e) the HR-TEM image of the red rectangle in (d); (f,g) HR-TEM images of some other h-BN nanosheet folded edges; (h) a possible single layer (marked by arrows) extruded from a thicker structure; (i) AFM images (dimensions: 200 nm 200 nm) and the corresponding height profiles of possibly monolayered functionalized h-BN nanosheet species; and (j) AFM images (dimensions: 400 nm 400 nm) and the corresponding height profiles of several few-layered functionalized h-BN nanosheet species.

nanosheets (using PEG-BN as the example, Figure 2a) were mostly irregularly shaped and of various lateral sizes ranging from a few tens of nm to as large as over 1 μm. At lower magnification, the relative thicknesses of the functionalized hBN nanosheets could be visually examined by the transparency/darkness of the nanosheets against the electron beam, with thinner h-BN nanosheets appearing lighter in color. The speed of centrifugation during the extraction process seemed to affect the thicknesses and the lateral sizes of the fractionated h-BN nanosheets.21 For example, the same PEG-BN sample (from ∼3000 g in a typical extraction process) was subjected to a higher centrifugation speed (∼10000 g). The resultant supernatant (Figure 2b) contained h-BN nanosheets mostly less than 300 nm in lateral sizes and appearing thinner, while the thicker nanosheets in the deposit (Figure 2c) were typically larger than 500 nm. The morphology of the h-BN nanosheets in ODA-BN (see Supporting


Information) was similar to that in PEG-BN but appeared generally thicker, which might be attributed to efficiency variation in the exfoliation induced, at least partially, by the difference in the sizes of the functional groups [molecular weight: 269.5 (ODA) versus 1500 (PEG)]. Similar to the graphene sheets,1 the h-BN nanosheets also tend to buckle and fold, making it possible to directly count the layers at the folded edges with high-resolution TEM (HR-TEM).19-21 For example, for the thin sheet shown in Figure 2d, enlargement revealed that the folded edge consisted of six layers (Figure 2e). Most of the h-BN nanosheets in the PEG-BN dispersion consisted of a few layers (Figure 2e-h), with the layer number typically in the range of 3-20 (or a thickness of ∼1-7 nm) when a centrifugation speed of ∼10000 g was used. The layer-layer distance was measured to be 0.33 nm, consistent with the theoretical value.

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transform infrared (FT-IR) spectra were present but much weaker in comparison to the functional group signals (e.g., alkyl C-H stretching at 2920 and 2850 cm-1 for ODA-BN) (see Supporting Information). Comparing to the supernatant fraction, the leftover residue from the extraction process still showed a small amount of weight loss (98%, 50 mg) and ODA (Aldrich, 90þ%, 500 mg) were mixed in a round-bottom flask and heated to ∼160-180 °C for 4-6 days under a steady nitrogen flow. After cooling the reaction mixture to room temperature, THF (∼15 mL) was added. The slurry was briefly sonicated and centrifuged (∼3000 g, 10 min), and the supernatant was collected. The extraction cycle was repeated 5-8 times on the residue from the centrifugation, and the supernatants were combined as the THF dispersion of the final product ODA-functionalized h-BN (or ODA-BN). 1H NMR (300 MHz, CDCl3): δ = 1.25 (b, 28H), 0.88 ppm (t, J = 6.6 Hz, 3H); 13C NMR (75 MHz, CDCl3): δ = 32.7, 29.7, 29.4, 22.7, 14.2 ppm. Similarly, PEG (Aldrich, Mr ∼ 1500) was used as the functional molecule in essentially the same procedure, except that deionized water was used as the extraction solvent. An aqueous dispersion of PEG-functionalized h-BN (or PEG-BN) was obtained as the product. 1H NMR (300 MHz, D2O): δ = 3.66 ppm (b); 13C NMR (75 MHz, D2O): δ = 71.8, 69.7, 60.4 ppm.

Figure 3. 2D 13C{1H} HETCOR NMR spectra of (a) ODA and (b) ODA-BN in room-temperature CDCl3. The corresponding 1H and 13C NMR spectra (acquired separately) were placed in the vertical and horizontal projections, respectively. Shown in the inset of (b) is a proposed cartoon structure of ODA-BN. The chemical structure of ODA with labeled H/C pair positions is also shown.

nanosheet species from large/thick parent particles. The average lateral sizes of the functionalized h-BN nanosheets were generally smaller than that in the original h-BN sample, suggesting that the functionalization/extraction process might be selective toward smaller h-BN species. We note that, despite the AFM results on the functionalized monolayered h-BN sheets discussed above, there was no conclusive spectroscopic evidence suggesting whether the amine-boron complexations preferentially occurred at or close to the defect sites or the edges of the nanosheets. The Lewis acid-base interactions of ODA or PEG with the h-BN surface are fundamentally different from the π-π


SUPPORTING INFORMATION AVAILABLE SEM and XRD of pristine h-BN, UV spectra of functionalized h-BN samples depicting the band gap position, microscopy images of ODA-BN, Raman spectra of ODA-BN after thermal treatments, FT-IR spectra of ODA-BN samples, 2D 13C{1H} HETCOR NMR spectra of PEG and PEG-BN, and a table for the room-temperature solubility values of the functionalized h-BN samples. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

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Corresponding Author: *To whom correspondence should be addressed. E-mail: yi.lin-1@ nasa.gov.

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ACKNOWLEDGMENT We thank Dr. W. Cao for experimental assistance in HR-TEM and D. Hartman for XRD measurements. Y.L. was partially supported by an appointment to the NASA Postdoctoral Program at the Langley Research Center, administered by ORAU through NASA Contract NNH06CC03B. T.V.W. was supported by the Langley Aerospace Research Summer Scholars (LARSS) Program.

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