Aqueous Soluble Boron Nitride Nanosheets Via Anionic Compound-Assisted Exfoliation

Materials Express

Aqueous soluble boron nitride nanosheets via anionic compound-assisted exfoliation

Fushen Lu1, ∗, Fei Wang1, Wenhua Gao1, Xiaochun Huang1, Xin Zhang1, and Yuliang Li2, ∗ 1Department of Chemistry, Shantou University, Guangdong 515063, P. R. China 2CAS Key Laboratory of Organic Solids, Beijing National Laboratory for Molecular Sciences (BNLMS), Institute of Chemistry, Chinese Academy of Sciences, Beijing 100190, P. R. China

ABSTRACT Few-layered boron nitride nanosheets (BNNSs) were prepared in presence of poly(sodium 4-styrenesulfonate) or sodium perylene-3,4,9,10-tetracarboxylate as dispersion and stabilization agents in water by exfoliation of commercial bulk h-BN powder. Taking advantages of the hydrophobic and/or -stacking interactions between BNNS surface and aromatic ringsIP: 192.168.39.210 in dispersionagents, On: Sat, the 25 resultingSep 2021 BNNS 20:12:03 dispersion was homogenous and stable for a month without precipitation.Copyright: Microscopic American andScientific spectroscopic Publishers results revealed that the bulk h-BN Delivered by Ingenta materials were intercalated and exfoliated down to 1–2 nm thick sheets. This study provided a facile and efﬁcient method to produce BNNSs in aqueous solution.

Article Keywords: Hexagonal Boron Nitride, Nanosheet, PSS, Exfoliation, Aqueous Dispersion.

1. INTRODUCTION structures and controlled properties still remains a signifi- cant and ongoing challenge. Hexagonal boron nitride (h-BN) is an isoelectronic and Nevertheless, several techniques, including chem- structural analogue of graphite in which the entire ical vapor deposition (CVD),12–20 micromechanical 2 sp carbons are replaced by alternating boron and cleavage,21 22 ball milling,23 24 and wet-chemical 1 2 nitride atoms. Single- or few-layered h-BN nanosheets exfoliation8 25–35 have been reported to produce BNNSs (BNNSs) possess excellent mechanical strength and ther- at diverse scales. Among various available approaches for mal conductivity that are comparable to their carbon BNNS production, wet-chemical exfoliation of bulk h-BN 1–4 sister system (graphene). However, BNNSs differ powder seems to be promising due to its cost-effectiveness, from graphene in other aspects such as large band versatile processability and easiness to scale up. For exam- gap (>5 eV), extreme oxidation resistance and high ple, Zhi et al. directly exfoliated bulk h-BN powder 1 2 chemical stability. These superior properties ren- into nanosheets (<20 layers) in NN-dimethylformamide der BNNS a promising material in a wide variety (DMF) for the fabrication of polymeric nanocomposites.8 5 of potential applications including electronics, optical However, the intercalation and exfoliation of bulk h-BN 6 7 8–10 devices, catalysis, and nanocomposites. In con- powder are rather difficult and the production yield of trast to extensive graphene research, studies on BNNSs BNNSs is generally low because of the strong “lip-lip” were less reported although single-layered h-BN sheets interactions between boron atoms in one layer and nitro- were produced almost a decade before the emergence gen atoms in AA stacked adjacent layers.1 2 Two strate- of graphene.11 The preparation of BNNSs with defined gies have been proposed to enhance the exfoliation of bulk h-BN powder. One is the use of dispersants or function- ∗Authors to whom correspondence should be addressed. alization agents during the exfoliation process to facil- Emails: [email protected], [email protected] itate and stabilize the BNNSs dispersion.25 26 Another

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Fig. 1. Scheme of exfoliation and functionalization of BNNSs. is the selection of an appropriate solvent or solvent powder) and 3,4,9,10-perylenetetracarboxylic dianhy- blends to minimize the energy of exfoliation (breaking the dride (98%) were supplied by Alfa Aesar. Poly(sodium lip-lip interactions).27 28 So far, many solvents such as 4-sytrenesulfonate) (Mw = 70,000) was purchased from isopropanol,27 ethanol-water,28 and basic water29 have Sigma-Aldrich. been examined for generating few-layered BNNSs. BNNS Sodium perylene-3,4,9,10-tetracarboxylate (SPTB) concentrations as high as 0.3 mg · mL−1 were obtained was synthesized by hydrolysis of 3,4,9,10- by methanesulfonic acid treatment.30 Generally speaking, perylenetetracarboxylic dianhydride. Brieﬂy, dianhydride exfoliation and solubilization of BNNSs in aqueous media (392 mg, 1 mmol) and sodium hydroxide (160 mg,

might be advantageous over organic solvents in terms of 4 mmol) were added to ethanol (60 mL). The mixture Article environmental friendliness.25 29 was sonicated and then refluxed for 2 h. Diethyl ether Water-soluble polymer electrolytesIP: 192.168.39.210 and aromatic On: Sat,(40 25 mL)Sep was2021 added 20:12:03 to the reaction, and the precipitate molecules have been widely used forCopyright: the functionaliza- American Scientificwas collected Publishers by filtration and dried in vacuum. tion and solubilization of graphene through hydrophobicDelivered by Ingenta and/or –stacking interactions between graphene and 2.2. Apparatus functionalization agents.36 The approach for graphene functionalization should apply to BNNS systems owing A bath sonicator (KQ 250DB, 250 W), a homogenizer to their dimensional similarity and isoelectronic struc- (Scientz XHF-D), and a benchtop centrifuge (Xiangzhi ture. For examples, water-soluble polyvinylalcohol and TG-16 W) were used in the exfoliation, dispersion, PEGylated phospholipid have been used to stabilize and other procedures. Atomic force microscopy (AFM) BNNSs and h-BN nanotubes, respectively.10 37 In the imaging was conducted on a multimode scanning probe work reported here, we chose an biocompatible anionic microscopy with a MultiMode Nanoscope IIIa controller. polymer—poly(sodium 4-styrenesulfonate) (PSS) and an Scanning electron microscopy (SEM) analyses were car- anionic aromatic compound—sodium perylene-3,4,9,10- ried out on a Hitachi S4800 field-emission microscope. tetracarboxylate (SPTB) to assist the exfoliation and func- Transmission electron microscopy (TEM) images were tionalization of BNNSs in water (Fig. 1). As expected, obtained from JEOL JEM-1011 or JEM-2011 micro- the hydrophobic aromatic rings in PSS and SPTB were scopes. The specimen for the imaging was prepared by adsorbed onto BNNSs surfaces while the anionic sulfonate depositing a few drops of a dilute suspension onto a and carboxylate groups sustained aqueous solubility. The holey carbon-coated grid, followed by solvent evaporation presence of functionalization agents during exfoliation under ambient conditions. Raman spectra were measured process indeed resulted in stable and homogenous BNNSs on Jobin-Yvon Labram HR 800 or Thermo Scientific DXR dispersion with sheet thickness down to 1–2 nm. This Raman spectrometers with a 780 nm laser. Optical absorp- process might open a way to solubilize BNNSs with tion and fourier transform infrared (FT-IR) spectra were biocompatible polymers and extend their applications in recorded on a 754C UV-visible spectrophotometer and a high-strength composites and biomedical fields. Nicolet Magna 750 spectroscopy, respectively.

2. EXPERIMENTAL DETAILS 2.3. Exfoliation of h-BN 2.1. Materials The exfoliation and dispersion of h-BN with PSS or STPB were carried out under the same experimental con- All chemicals and reagents were used without further ditions and procedures. In general, as-supplied h-BN pow- puriﬁcation. Hexagonal boron nitride (325 mesh der (200 mg) was suspended in deionized water (200 mL)

Mater. Express, Vol. 3, 2013 145 Materials Express Aqueous soluble boron nitride nanosheets via anionic compound-assisted exfoliation Lu et al.

via homogenization and sonication for 10 h. PSS or SPTB (200 mg) was added to the h-BN suspension and sonicated for another 2 h. The mixture was heated to 80 C for 5 h and then cooled to room temperature under vigorous stirring. The resulting solution was centrifuged at 3,000 rpm for 30 min to remove the large h-BN pel- lets in the sediment. The supernatant was centrifuged at ultrahigh speed or ﬁltered through a cellulose ester ﬁlter membrane (0.22 m pore size), and the solid was retained and repeatedly washed with deionized water to remove free PSS or SPTB.

3. RESULTS AND DISCUSSION The as-supplied h-BN powder consists of crystalline ﬂakes with lateral size of 1–5 m and thickness of a few hundred nanometers (Fig. 2(a)). The pristine h-BN powder was hardly dispersible in water due to their hydrophobicity. Sonicating h-BN powder alone (without dispersion agents) in neutral water resulted in unstable h-BN suspension which precipitated within hours. In contrast, h-BN could be well dispersed in aqueous media by adding poly(sodium 4-styrenesulfonate) during the exfoliation process. The resultant “milky” but transparent PSS-BNNS dispersion was stable for a month without precipitation. The mor- phologies of dispersed h-BN nanosheetsIP: 192.168.39.210 were visualized On: Sat, 25 Sep 2021 20:12:03 by transmission electron microscopyCopyright: (TEM). Typically, American Scientific Publishers the lateral sizes of BNNSs were of the order ofDelivered microns by Ingenta and seemed to be transparent against the electron beam (Fig. 2(b)), indicating that the morphological features Article of PSS-functionalized sheets were similar to those few- layered sheets prepared by other approaches.27 Selected area electron diffraction (SAED) pattern (Fig. 2(b) inset) conﬁrmed that the crystalline structures of original h-BN powder were maintained in the exfoliation process. The high resolution TEM image of a typical crystalline BNNS revealed that this sheet consisted of three layers (Fig. 2(c)). The thickness and morphology of BNNSs were further characterized by AFM imaging. The height of an indi- vidual micron-sized sheet was measured to be ∼2nm (Fig. 3(a)). Based on the theoretical interlayer distance (0.333 nm), this sheet consisted of six layers. However, the actual number of layers should be fewer since the trapped solvents between a wet-exfoliated BNNS and the underlying substrate might make AFM height of monolayer sheet close to 1 nm.2 26 29 Thinner h-BN sheets with height of 1–2 nm were also found in the AFM images. However, these sheets showed irregular shapes and their lateral sizes Fig. 2. (a) SEM images of bulk h-BN powder; (b) a representative were dramatically reduced to the range of 100–500 nm TEM image of PSS-BNNS and the SAED pattern; (c) a high-resolution (Fig. 3(b)). TEM image of PSS-BNNS. Fourier transform infrared (FT-IR) spectra of PSS- BNNS, starting h-BN powder and pure PSS were col- 1373 cm−1 which correlated with the bending and shear- lected to qualitatively identify the interaction between ing vibrations of B–N bonds, respectively. The absorp- PSS and h-BN sheets. As shown in Figure 4(a), h-BN tion peaks of pure PPS at 1007, 1041 and 1131 cm−1 powder exhibited two absorption peaks at 818 and could be considered as the characteristic absorptions of

146 Mater. Express, Vol. 3, 2013 Aqueous soluble boron nitride nanosheets via anionic compound-assisted exfoliation Materials Express Lu et al.

Fig. 3. Representative AFM images of PSS-BNNS and a height proﬁle along the line. (a) A micron-sized BNNS; (b) smaller BNNSs.

–O S O– stretching vibrations.38 The absorption between the BNNS surface and aromatic rings of PSS spectrum of PSS-BNNS was close to a superposition from was likely responsible for the binding (– stacking and those of the underlying h-BN and PSS. No new chemical hydrophobic interactions), which was commonly observed bond was detected. Therefore, the edge-to-face interaction in the graphene-PSS systems.39 The UV–vis absorbance spectra of pure PSS and PSS-BNNS were shown in Figure 4(b). Both curves fea- tured three PSS peaks at 200, 228, and 265 nm. How- ever, the absorption peak of BNNSs at 204 nm was not Article distinguished because of its overlap with a PSS peak. IP: 192.168.39.210 On: Sat,Although 25 Sep 2021 UV-vis 20:12:03 and FT-IR spectra clearly showed the Copyright: American Scientificco-existence Publishers of BNNSs and PSS in the PSS-BNNS sam- Delivered by Ingenta ple, the exact ratio of these two constituents was hardly identified. The usually effective thermogravimetric anal- ysis (TGA) was not applicable to the estimate of h-BN content in PSS-BNNS sample because the PSS polymer could not be completely removed in the TGA scans. The compositions in PSS-BNNS samples were determined through inductively coupled plasma atomic emission spec- trometry (ICPE-9000, Shimadzu), in which the content of elemental sulfur was measured and correlated with the standard PSS content curve. The PSS content in the PSS-BNNS sample was estimated to be ∼50 wt%. In order to calculate the maximum solubility of BNNSs in aqueous media, the water in an as-prepared PSS-BNNS sample was evaporated until the appearance of BNNS precipitates. The concentration of saturated PSS-BNNS solution was measured to be 0.32 mg · mL−1, corresponding to BNNS equivalent concentration of 0.16 mg · mL−1. The extinction coefficient of PSS-BNNS at 500 nm was roughly estimated to be 2.8 L g−1 cm−1 since its absorption obeyed Lambert-Beer’s Law and PSS did not absorb in the visible region. The accuracy of the obtained coefficient value might be influenced by light scattering of micron-sized h-BN nanosheets though this number is consistent with the reported extinction coefficient for octadecylamine- 26 Fig. 4. (a) FT-IR spectra of bulk h-BN powder (dash line), PSS (dash- functionalized h-BN nanosheets sample. dot line) and PSS-BNNS (solid line); (b) UV-vis absorption spectra of Motivated by the successfully “peeling” h-BN powder PSS-BNNS (solid line) and PSS (dash-dot line) and a photograph of into few-layered sheets with water-soluble polymer, PSS-BNNS dispersion (inset). we extended the dispersion agents to small aromatic

Mater. Express, Vol. 3, 2013 147 Materials Express Aqueous soluble boron nitride nanosheets via anionic compound-assisted exfoliation Lu et al.

molecules (SPTB), exploring the same – stacking and hydrophobic interactions. The water-soluble SPTB was quantitatively synthesized by hydrolysis of 3,4,9,10-perylenetetracarboxylic dianhydride. Adopt- ing the same experimental conditions and procedures as those for PSS-BNNS, a homogenous and stable of SPTB- functionalized BNNS (SPTB-BNNS) was obtained. The

Fig. 6. Raman spectra of bulk h-BN powder (solid line) and PSS-BNNS (dashed line). IP: 192.168.39.210 On: Sat, 25 Sep 2021 20:12:03 Copyright: American Scientific Publishers Delivered by Ingenta size and thickness of SPTB-BNNS are rather similar to those of PSS-BNNS sample. A TEM image of a micron-

Article sized SPTB-BNNS sheet was shown in Figure 5(a). Accordingly, the height of SPTB-BNNS was estimated to be ∼2 nm from AFM images (Figs. 5(b–c)). The SPTB molecules likely aggregated on the BNNS surfaces instead of forming uniform coating layer, which might be attributed to the strong H- or J-type stacking effects between planar perylene themselves.40 We further characterized the exfoliated BNNSs with Raman spectroscopy. However, the detection of Raman shift for SPTB-BNNS sample was hardly achievable since the overwhelming ﬂuorescence background from SPTB compounds wiped all the Raman features. On the con- trary, PSS-BNNS sample provided the characteristic peak at 1365 cm−1 which belongs to the B–N high-frequency

vibration mode (E2g (Fig. 6). Under the same measure- ment condition, the peak for exfoliated sheets became much weaker and showed red shift of 4 cm−1 in compar- ison with the peak at 1369 cm−1 for bulk h-BN powder. According to the literature report, this red shift was usually caused by random strain dominated in bilayer.22

4. CONCLUSION

Fig. 5. (a) A representative TEM image of SPTB-BNNS; (b, c) AFM BNNSs were produced by a facile but effective images of SPTB-BNNS and a height proﬁle along the line. solution-based method. Namely, commercial bulk h-BN

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Received: 5 February 2013. Revised/Accepted: 22 March 2013.

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