- Nano Idea
- Open Access
High-Efficient Liquid Exfoliation of Boron Nitride Nanosheets Using Aqueous Solution of Alkanolamine
© The Author(s). 2017
- Received: 20 September 2017
- Accepted: 6 November 2017
- Published: 17 November 2017
As one of the simple and efficient routes to access two-dimensional materials, liquid exfoliation has received considerable interest in recent years. Here, we reported on high-efficient liquid exfoliation of hexagonal boron nitride nanosheets (BNNSs) using monoethanolamine (MEA) aqueous solution. The resulting BNNSs were evaluated in terms of the yield and structure characterizations. The results show that the MEA solution can exfoliate BNNSs more efficiently than the currently known solvents and a high yield up to 42% is obtained by ultrasonic exfoliation in MEA-30 wt% H2O solution. Finally, the BNNS-filled epoxy resin with enhanced performance was demonstrated.
- BN nanosheets
- Liquid exfoliation
Since the discovery of graphene in 2004, the interests in graphene and its analogues two-dimensional materials [1, 2] are ever growing throughout the world. Single-layer or few-layer boron nitride nanosheets (BNNSs), known as “white graphene,” share near identical structure to graphene that the sp2 hybridized B and N atoms covalently bind into hexagonal crystal in single layers, resulting in weak van der Waals forces between them. Due to the structure and a wide band gap (5.5 eV) , BNNSs are endowed with outstanding mechanical, thermal, and dielectric properties, as well as excellent chemical stability, thus exhibiting great potentials in applications such as transparent films [3, 4], protective coating [5, 6], advanced composites [7–9], dielectrics [10, 11], and electronic devices [12, 13] etc.
To produce ultra-thin BNNSs, a variety of methods such as ball milling [14–16], intercalation-oxidation [17, 18], chemical vapor deposition (CVD) [3, 4], and liquid exfoliation [2, 19–28] have been developed. Of these methods, both ball milling and intercalation-oxidation methods are time-consuming and prone to induce impurity and defects in samples, while the CVD costs highly and is applied to prepare continuous film instead of disperse nanosheets that are more popular in practical applications. Recently, liquid exfoliation of BNNSs from hexagonal boron nitride (hBN) powder has received much attention because it is easy to use, economical, free of defect, etc. The driving forces were ascribed dynamically to sonic vibration [19, 20] or liquid shear [21, 22], and thermodynamically to minimization of Gibbs mixing free energy [23, 24] or interfacial energy  between the nanosheets and solvents. According to the latter, the composition and properties of used solvent play an important role in liquid exfoliation. Much research has shown that hBN can be exfoliated preferentially in few pure solvents such as N-methyl-2-pyrrolidone (NMP), dimethylformanmide (DMF), and isopropanol (IPA) [9, 19–24, 26] and some mixed solvents [25, 27–29]. However, high-efficient and cheap solvents for hBN liquid exfoliation have been rarely reported, limiting the large-scale preparation and applications of BNNSs.
In the present paper, monoethanolamine (MEA) aqueous solution was attempted for the first time for liquid exfoliation of BNNSs. It was found to exfoliate hBN more efficiently than the other solvents with very high yield. Moreover, this solution has higher specific surface tension (SST) than that of known solvents. The obtained BNNSs were characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM), Raman, and X-ray photoelectron spectroscopy (XPS) techniques. Finally, as an example, the BNNSs were employed to reinforce epoxy resin (ER). The obtained composites exhibited improved thermal and mechanical properties.
Now two problems arise from above observations: first, how to understand the superior exfoliation performance of MEA compared with other solvents; second, why can the introduction of water in appropriate amount in MEA improve the exfoliation? Regarding the first problem, we resort to the solubility parameter theories (SPTs). Following these theories, Coleman et al. [23, 24] suggested that the solvents for effective exfoliation were those with dispersive, polar, and H-bonding solubility parameters matching those of layered materials in order to minimize the exfoliation energy. They found that hBN was most effectively dispersed in those solvents with a SST close to 40 mJ/m2. In other studies [16, 28], this value was reported as 20~40 mJ/m2. In our system, pure MEA has a SST of 44.8 mJ/m2 according to ref.  (where the data at 50 °C was taken according to the experimental condition, the same below), which roughly agrees with this case. However, when MEA is mixed with 20~60 wt% of water (the second problem), an enhanced exfoliation was observed in this solution, whose SST is about 49~55 mJ/m2 (Fig. 2a) and much higher than the previous values. This enhanced exfoliation which occurred in high SST of mixed solvents is presumably due to following factors: (1) MEA molecules that tend to form a network or ring-like structure due to the interactions among amino and hydroxyl groups are disaggregated by the added water molecules , allowing them to intercalate BN layers more easily and enhance the exfoliation; (2) the water makes the amino groups of MEA absorbed on BNNSs hydrolyzed and increases surface potential of the BNNSs, hence introducing an additional electrostatic stability, as observed in MEA-30 wt% H2O-exfoliated BNNS suspension (Fig. 2b); (3) more or less addition of the water would deviate from the condition above and restrict the liquid exfoliation.
In summary, we reported on MEA aqueous solution as a new type of mixed solvents for high-efficient and cost-effective liquid exfoliation of BNNSs. The control experiments show MEA can exfoliate hBN superior than currently known solvents, and this ability can be further improved by the addition of water of appropriate amount in MEA. In the optimum, an exfoliation yield more than 40% was achieved in MEA-30 wt% H2O solution. Also, we found that this solution, when resulting in the most efficient exfoliation of BNNSs, has a much high SST which deviated greatly from the predictions by SPTs, suggesting that additional interactions might need to be considered in SPTs to better interpret the liquid exfoliation. The exfoliated BNNSs demonstrate an ability of significantly improving the thermal and mechanical properties of polymers. The mixed solvent here reported enables the scalable exfoliation and applications of BNNSs and exhibits great potentials in other exfoliation techniques, such as shear exfoliation and ball milling exfoliation, and other two-dimensional materials.
hBN powder (1~5 μm, 99.5%), monoethanolamine (MEA), N-methyl-2-pyrrolidone (NMP), isopropanol (IPA), dimethylformanmide (DMF), tert-butanol (tBA), polyvinylpyrrolidone (PVP, molecular weight ~8000), methylhexahydrophthalicanhydride (MeHHPA), and 2,4,6-tris(dimethyl -aminomethyl) phenol (DMP-30), purchased from Aladdin industrial corporation of Shanghai, were of reagent grade. Bisphenol-A epoxy resin (epoxide number 0.48~0.54) was provide by Baling Company, SINOPEC.
Preparation of BNNSs
Typically, 200 mg of pristine hBN powders was mixed in 50 mL of MEA or MEA aqueous solution with given water content in a 200-mL beaker, before sonicated for 4 h at about 50 °C in a 6-L bath sonicator (KQ3200DA, Kunshan Shumei) operating at 40 kHz and supplied power dissipation of 150 W. The resultant suspension was centrifugated at 3500 rpm for 20 min. The supernatant was decanted to afford a concentrated solution of exfoliated BNNSs. It was washed with ethanol repeatedly and vacuum dried at 100 °C overnight, giving the BNNS powder. The yield is defined as the mass ratio of exfoliated BNNSs to pristine hBN. For comparison, several popular solvents such as NMP, DMF, IPA, and tBA were chosen to exfoliate hBN powders following the same process.
Preparation of ER-BNNS Composite
First, 30 mg BNNS powder and 100 mg PVP were dispersed in 10 mL of DMF. Then the dispersion was stirred at 100 °C for 6 h, allowing PVP to adhere to the surfaces of BNNSs. The resultant suspension was separated, washed, and dried following the above procedure, giving PVP-functionalized BNNSs (BNNSs-PVP). Third, bisphenol-A epoxy resin, MeHHPA, and BNNSs-PVP (41:57.5:1 by mass ratio) were mixed for 40 min before being vacuum degassed at 60 °C for 20 min; the mixture was added with 0.5% DMP-30 as the promoter and then sonicated for 10 min. Finally, the resulting paste was casted in a mold and cured using a heating procedure: 80 °C/10 h + 100 °C/3 h + 150 °C/3 h, forming the ER-1% BNNS composite sheets as testing samples. For comparison, pure ER samples were obtained using the above process in the absence of BNNSs-PVP. The samples are tape-like (DMA test) with a size 10 mm × 25 mm × 1 mm or dumbbell-like (mechanical test) with thickness of 1 mm.
Optical absorption spectra were taken from a spectrophotometer (UV-vis; Persee T1910). The chemical components were analyzed using Fourier transformation infrared spectroscopy (FTIR; Bruker IFS66V), Raman spectrometry (RS; HORIBA JY, LabRAMXploRA ONE), and X-ray photoelectron spectroscopy (XPS; Kratos Axis Supra, Al-Kα radiation). The phases were identified by X-ray diffraction (XRD; PANalytical, X’Pert PRO, Cu-Kα radiation, 1.54 Å). The morphology and size of nanosheets were observed using field-emission scanning electron microscopy (SEM; Hitachi, S4800), transmission electron microscopy (TEM; JEOL, JEM-2010), and atomic force microscopy (AFM; Bruke, Dimension Icon). For ER/BNNS samples, dynamic mechanical analysis was carried out with a dynamic mechanical analyzer (DMA; DMA8000, Perkin Elmer) based on a single cantilever mode at a frequency of 1 Hz. Tensile strength and Young’s modulus were measured using an electronic universal testing machine (CMT-200, Jinan Liangong) with a load range of 0~200 kN.
We would also like to thank Zemin Yuan, doctoral student in the Department of Functional Materials Research, Central Iron and Steel Research Institute of China, for his assistance with the TEM and AFM analyses.
The authors are grateful for the financial support of the National Natural Science Foundation of China under grant 51462028.
Availability of Data and Materials
Boron nitride nanosheets (BNNSs), monoethanolamine (MEA), epoxy resin (ER), N-methyl-2-pyrrolidone (NMP), isopropanol (IPA), dimethylformanmide (DMF), tert-butanol (tBA), polyvinylpyrrolidone (PVP), methylhexahydrophthalicanhydride (MeHHPA), 2, 4, 6-tris(dimethyl-aminomethyl) phenol (DMP), Fourier transformation infrared spectroscopy (FTIR), Raman spectrometer (RS), X-ray photoelectron spectra (XPS), X-ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM), atomic force microscopy (AFM).
BZ and QW conceived the project and designed the experiments. BZ, HS, and HY carried out the experiments and characterizations. BZ, RL, XG, and RX took part in the analysis of the results, BZ and HY wrote the paper, and all of the authors read and revised the paper. All authors read and approved the final manuscript.
The authors declare that they have no competing interests.
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