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Scalable exfoliation and dispersion of few-layer hexagonal boron nitride nanosheets in NMP-salt solutions
Applied Surface Science 488 (2019) 656–661
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Scalable exfoliation and dispersion of few-layer hexagonal boron nitride nanosheets in NMP-salt solutions ⁎
T
⁎
Huiyong Wang , Xin Su, Tao Song, Zhiyong Li, Yuling Zhao, Hao Lou, Jianji Wang
Henan Key Laboratory of Green Chemistry, Collaborative Innovation Center of Henan Province for Green Manufacturing of Fine Chemicals, Key Laboratory of Green Chemical Media and Reactions, Ministry of Education, School of Chemistry and Chemical Engineering, Henan Normal University, Xinxiang, Henan 453007, PR China
A R T I C LE I N FO
A B S T R A C T
Keywords: Hexagonal boron nitride Liquid-phase exfoliation NMP-salt solutions Interactions
Efficient and large-scale production of boron nitride (h-BN) nanosheets is very important for their applications. Here, we have developed a facile strategy for liquid-phase exfoliation of h-BN nanosheets in organic electrolyte solutions where N-methyl-pyrrolidinone (NMP) is used as an organic solvent, and sodium citrate, sodium tartrate, ammonium oxalate, potassium sodium tartrate or ethylenediaminetetraacetic acid disodium salt is used as a salt. It is found that the dispersion content of h-BN nanosheets has been greatly improved by using organic electrolyte solutions. For example, by addition of ethylenediaminetetraacetic acid disodium salt into NMP, the content of the exfoliated h-BN is 36 times that in neat NMP. Furthermore, approximately 85% of the h-BN nanosheets obtained in such a strategy is 1–4 layers. Mechanism study indicates that the strength of Lewis basicity of anion and size of cation of the salts play an important role in the exfoliation of h-BN by organic electrolyte solutions. The liquid exfoliation strategy reported here shows great promise for the mass production of high-quality few-layer h-BN nanosheets.
1. Introduction Two dimensional (2D) nanosheets such as graphene, hexagonal boron nitride (h-BN), and transition metal dichalcogenides have been receiving extensive attention in recent years, due to their ultrathin structure and outstanding properties [1,2]. The merits of these materials make them great potential in a huge range of applications in scientific and technological fields. Among these 2D nanosheets, the h-BN nanosheets, so called “white graphene”, has also been intensively studied in recent years because of their advantages compared to graphene. In addition to the unique chemical and thermal stability [3–5], h-BN nanosheets also show great mechanical strength and electrical insulating properties (band gap of 5–6 eV) [6,7]. Therefore, h-BN nanosheets have wide applications in sensors, energy storage devices, temperature antioxidant materials, composite materials and other fields [8]. To promote large-scale applications of h-BN nanosheets, the production of single or few layers of h-BN nanosheets (< 5 layers) in large quantity and high quality become very important. Much effort has been devoted to develop various preparation methods for h-BN nanosheets. To date, h-BN nanosheets have been prepared via chemical vapor deposition, segregation methods, and mechanical or sonication exfoliation approaches [9–15]. The chemical vapor deposition and segregation methods can provide single-layer h-BN nanosheets, but usually need ⁎
extreme conditions of temperature and pressure. On the other hand, mechanical or sonication exfoliation method is usually high energy/ time consuming. Therefore, they are not widely considered as suitable to synthesize h-BN nanosheets on larger scales. Liquid-phase exfoliation is one of the most potential methods for preparing h-BN nanosheets with high quality from bulk h-BN powder on large scales. In general, h-BN powder is mixed with a solvent, and ultrasonic energy is introduced into the system in the liquid-phase exfoliation. To obtain high yield of exfoliated h-BN nanosheets, good candidate solvents possessing the ability to minimize the enthalpy of the whole mixture and the energy for exfoliation are preferred. Various solvents have already been investigated for the dispersion and exfoliation of h-BN using sonication in the literatures [16–22], such as isopropyl alcohol, N, N-dimethylformamide, dimethyl sulfoxide, and NMP. To improve dispersion and exfoliation of h-BN, there are also some successful attempts to combine two or more solvents during sonication [23,24]. However, only 5% -10% yield could be achieved by using these solvents. In addition, compared to graphene, the partial ionic character in the BeN bond of h-BN and the “lip-lip” interactions between neighboring layers make the exfoliation of h-BN bulk more difficult [25]. Therefore, the liquid exfoliation of bulk h-BN is even more challenging than other layered materials, and more efforts should be devoted to increasing yield of the exfoliated h-BN nanosheets and
Corresponding authors. E-mail addresses: [email protected] (H. Wang), [email protected] (J. Wang).
https://doi.org/10.1016/j.apsusc.2019.05.296 Received 11 March 2019; Received in revised form 23 May 2019; Accepted 25 May 2019 Available online 27 May 2019 0169-4332/ © 2019 Elsevier B.V. All rights reserved.
Applied Surface Science 488 (2019) 656–661
H. Wang, et al.
diffractometer (D8&Advance, Bruker) with monochromatic Cu Kα radiation (λ = 1.5418 Å). AFM analysis was carried out in an AFM instrument controlled with the Nanoscope V software V8 (Bruker Corporation, UK). Raman spectra were recorded on a HORIBA JobinYvon XploRA spectrophotometer with 633 nm excitation. Yield of the exfoliated h-BNNSs was calculated from the following equation:
the dispersion content. In principle, due to the electron deficient behavior of B atoms, h-BN owns Lewis acid characteristics and is vulnerable to attack from Lewis base compounds, especially at the defect sites in h-BN [25]. It is conceivable that if a Lewis base compound is added in a solvent, a Lewis acid-base complex can be formed, which may help to overcome the interaction between layers of h-BN, and consequently large amount of h-BN nanosheets would be easily exfoliated from h-BN bulk with the assistance of vigorous sonication. In addition, the presence of Lewis acid-base complexes will promote the dispersion of h-BN nanosheets and enhance the dispersion content and yield of h-BN nanosheets. In order to verify this concept in this work, we report liquid exfoliation of h-BN nanosheets in five kinds of binary organic electrolyte solutions. For this purpose, NMP is chosen due to the fact that it is the best solvent for the exfoliation of h-BN nanosheets. At the same time, five kinds of salts including sodium citrate, sodium tartrate, ammonium oxalate, potassium sodium tartrate, and ethylenediaminetetraacetic acid disodium salt are selected as intercalators to examine the relationship between the structures of salts and the exfoliation performance. The optimization of the exfoliating process is studied systematically, and hBN nanosheets have been characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM), atomic force microscopy (AFM), Raman and ultraviolet visible absorption spectroscopy. It is found that the h-BN nanosheets are predominantly present in 1–4 layers, and the addition of salts significantly increased exfoliation efficiency and dispersion content of the h-BN nanosheets. The dispersion content (up to around 1.8 mg mL−1) is 36 times that in neat NMP, and about 6 times the best value reported in organic solutions.
Yield (%) = (weight of h − BN nanosheets) × 100/(weight of h − BN bulk) (1)
3. Results and discussion 3.1. Optimization of exfoliation efficiency Here, five different salts were used to examine the effect of chemical nature of the salt on the exfoliation performance of the h-BN powder in NMP. It was found that among the salts investigated in this work, only sodium citrate, sodium tartrate, and ethylenediaminetetraacetic acid disodium salt could significantly increase the yield of h-BN nanosheets in NMP solvent. Therefore, these three kinds of salts were taken to examine salt effect in exfoliation efficiency of bulk h-BN powder. To maximize the exfoliation efficiency of h-BN powder, it is necessary to optimize the experimental conditions for the preparation of hBN nanosheets. In doing so, the effect of the starting h-BN mass, salt concentration, sonication time, and system temperature on the dispersed content and yield of h-BN nanosheets was investigated in detail. To conveniently determine the optimized conditions, the absorbance values of h-BN nanosheets dispersions were used to estimate the concentrations of h-BN dispersion solution, which is based on the LambertBeer law. Figs. S1–S4 (Supporting information) show the UV–vis data for h-BN nanosheets dispersion prepared under different experimental conditions. Because h-BN and salts do not exhibit any observable absorption peaks, a wavelength of 300 nm was used to compare the relative absorbance between samples. All data are the averaged results of three trials for each system. Taking organic electrolyte solution containing ethylenediaminetetraacetic acid disodium as an example, it was found that the optical absorbance of h-BN nanosheets in solution was almost unchanged with the variety of the system temperature from 15 to 75 °C (as shown in Fig. S1). So, 25 °C was chosen in this work. Furthermore, the optical absorbance of h-BN nanosheets in solution was firstly increased and then decreased with the increase of ethylenediaminetetraacetic acid disodium salt content. Therefore, the optimum concentration of ethylenediaminetetraacetic acid disodium salt in NMP was 20 mg mL−1. At last, the absorbance values of the h-BN dispersions increased rapidly with the increase of the starting h-BN mass and sonication time up to 12 mg mL−1 and 12 h, respectively, and then kept at an almost constant absorbance. Thus, the optimal starting h-BN mass and sonication time was 12 mg mL−1 and 15 h, respectively. The optimized conditions for the preparation of h-BN nanosheets in different organic electrolyte solutions were listed in Table 1. The content of these final h-BN nanosheets dispersions was estimated using the optical absorption spectrum and the Lambert-Beer law, which was presented in Table 1. The yield of the as-obtained h-BN nanosheets was calculated from Eq. (1), and the results were shown in Table 1. It can be seen that compared to neat NMP, the concentrations and yields of h-BN nanosheets in organic electrolyte solutions were effectively improved. The content of h-BN nanosheets dispersion prepared in NMP-ethylenediaminetetraacetic acid disodium salt (sodium citrate or sodium tartrate) solutions was 36, 22 and 11 times that in neat NMP, and the yield of hBN nanosheets was increased by 36.5, 21.5, and 10.5 times, respectively. Interestingly, the dispersion content of h-BN nanosheets in ethylenediaminetetraacetic acid-NMP solution was around 6 times that obtained in hyperbranched polyethylene + CHCl3 solution, [26] which provides the best yield for the liquid exfoliation of h-BN up to now. It
2. Experimental 2.1. Materials Hexagonal boron nitride (h-BN, 98.5%, 1 μm particle size), N-methyl-pyrrolidinone (99%), sodium citrate (98%) and sodium tartrate (99%) were purchased from Aladdin Co., Ltd. (Shanghai, China). Ammonium oxalate (99.8%), potassium sodium tartrate (99.5%), and ethylenediaminetetraacetic acid disodium salt (99%) were obtained from Shanghai Macklin biochemical Co., Ltd. These chemicals were directly used without further purification. 2.2. Methods A given quantity of the bulk h-BN powder was mixed with binary organic electrolyte solutions. Then, the mixture was added into a centrifuge tube and sonicated in a bath sonicator (KQ-400GKDV) at a power of 400 W, and the sonicator was equipped with a temperature control system to prevent overheating during sonication. After sonication, the exfoliated solution was centrifuged at 4000 rpm, relative centrifugal force (RCF) = 1644 g, for 40 min to remove the un-exfoliated h-BN. Subsequently, the top 2/3 of the dispersion was gathered carefully, centrifuged at 10000 rpm (RCF = 10,278 g) for 10 min, and the precipitate was collected. To remove the salt, the precipitate was washed by 10 mL water under sonicating for 5 min, then centrifuged at 5000 rpm (RCF = 2569 g) for 5 min, and re-gathered. The rinsing process was repeated for four times. 2.3. Characterization The concentration of h-BN nanosheets was determined by UV–Vis spectrometer (TU-1900) at 300 nm with corresponding organic electrolyte solutions as blanks. SEM image was performed on field-emission SEM system (Hitachi SU8010). The TEM image was obtained using a JEM 2100 electron microscope with the accelerating voltage of 200 kV. XRD patterns of h-BN nanosheets were acquired by X-ray powder 657
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Table 1 The optimized conditions for the preparation of h-BN nanosheets in different salt solutions of NMP together with the corresponding dispersion concentration and yield of h-BN nanosheets. Salt
Ethylenediaminetetraacetic acid disodium Sodium citrate Sodium tartrate Neat NMP
Csalt (mg mL−1)
Ch-BN (mg mL−1)
Sonication time (h)
Dispersion content (mg mL−1)
Yield (%)
20 23 10 0
12 12 12 12
15 15 15 15
1.80 1.08 0.56 0.05
15 9 4.7 0.4
Fig. 1. SEM images of the bulk h-BN powder and the h-BN nanosheets prepared in ethylenediaminetetraacetic acid disodium salt-NMP solution.
bulk h-BN powder also exhibited distinct differences. h-BN nanosheets had an ultrathin, transparent and overlapped flat structure, whereas a bulky and opaque structure was observed for the parent h-BN, which provides clear evidence that the h-BN sheets were exfoliated thoroughly into very thin layers. In addition, SAED pattern of the h-BN nanosheets prepared in NMP-salts solutions illustrated the hexagonally symmetric structure of h-BN. [27] These results suggest that the crystal structure of h-BN nanosheets was not destroyed during sonication process and their single crystalline was retained. The specific lateral dimensions and thickness of the exfoliated h-BN nanosheets were analyzed by atomic force patterns. Larger than 100 hBN nanosheets pieces were measured for each sample. The detailed statistical analysis from the AFM images of h-BN nanosheets was performed and a histogram of the thickness distribution from AFM images was plotted (Figs. 3 and S6). It was found from Fig. 3 that the thickness of approximately 85% h-BN nanosheets prepared in this work was
can be seen clearly that the dispersion content of the as-exfoliated h-BN nanosheets reported in this work is significantly higher than that reported previously. 3.2. Morphology and quality of the exfoliated h-BN nanosheets In order to illustrate the morphology of exfoliated h-BN nanosheets, scanning electron microscopy (SEM) were used. Fig. 1 shows the SEM images of the bulk h-BN powder and the exfoliated h-BN nanosheets. Compared with the bulky h-BN precursors, the h-BN nanosheets demonstrate a much smaller size and a nanosheet-like morphology. Figs. 2 and S5 exhibited typical TEM images and corresponding selected area electron diffraction (SAED) patterns of the h-BN nanosheets and parent bulk h-BN powder. It was found that the irregular shaped h-BN nanosheets with lateral sizes ranging from a few tens of nanometers to as long as over 100 nm. The TEM images of h-BN nanosheets and parent
Fig. 2. TEM image (a) and corresponding SAED pattern (b) of the h-BN nanosheets prepared in ethylenediaminetetraacetic acid disodium salt-NMP solutions. 658
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Fig. 3. AFM image and the layer number distribution of h-BN nanosheets prepared in ethylenediaminetetraacetic acid disodium salt-NMP solutions.
0.4–1.8 nm. Given that the thickness of a single-layered h-BN is about 0.4–0.5 nm, [10] this observation indicates that the as-prepared h-BN nanosheets was mostly constitutive of 1–4 atomic layers. This result suggested that using NMP-salt solution is a promising strategy for the exfoliation of few-layer h-BN nanosheets. Moreover, the nanosheets showed lateral dimensions about 100 nm, which is consistent with that observed from TEM. To further characterize the crystalline structure of the exfoliated hBN nanosheets, XRD measurements were performed, and the results for the bulk h-BN powder and the exfoliated h-BN nanosheets in different NMP-salts solutions were shown in Fig. 4. It can be seen from Fig. 4 that XRD finger print peak of the bulk h-BN powder was present at 26.75°, 41.7°, 55.3°and 75.9°, which are assigned as its (002), (004), (100), and (101) plane, respectively. This result indicates a typical hexagonal structure of h-BN (JCPDS card no. 01-073-2095). It is known that the broadness of the peak of (002) plane has been widely used for the analysis of single or fewer layered graphene. Similarly, thickness of the exfoliated h-BN nanosheets can also be determined based on the calculation of full width of half maximum of the peak of (002) plane. The
full width of half maximum of the peak of (002) plane for the exfoliated h-BN nanosheets in different NMP electrolyte solutions was broader (∼0.08°) compared to the bulk h-BN. The broadness of the diffraction peak of (002) plane signifies the nanoscale thickness nature (∼2 nm thickness calculated by Scherrer equation). This result confirms the thickness characterization of the exfoliated h-BN nanosheets observed from TEM technique. In addition, the intensity of the peak of (002) plane for h-BN nanosheets significantly decreased and two-theta of this peak slightly downshifted from 26.75 to 26.65 °C, corresponding to an increase in the interplanar distance from 0.33 nm to 0.34 nm computed from Bragger equation. The increased interplanar distance and the decreased intensity of other diffraction peaks [(100), (104) and (110)] suggest the formation of ultrathin h-BN nanosheets with a less extended/ordered stacking in the c direction. The thickness of the h-BN nanosheets and their crystalline structures information were also provided by Raman spectroscopy measurements according to the shift and broadness of the Raman peaks. As shown in Fig. 5, the Raman spectra of bulk h-BN and h-BN nanosheets exhibited a
Fig. 4. XRD patterns of the bulk h-BN powder and the as-exfoliated h-BN nanosheets in different NMP-salts solutions.
Fig. 5. Raman spectra for the bulk h-BN powder and the exfoliated h-BN nanosheets prepared in different NMP-salts solutions. 659
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ethylenediaminetetraacetic acid disodium salt > sodium citrate > sodium tartrate > ammonium oxalate, which was approximately consistent with the order of pKa values of the corresponding acid in water (6.16, 6.4, 4.37, 4.19 for ethylenediaminetetraacetic acid, citrate acid, tartrate acid and oxalate acid, respectively). [31] This indicates that the exfoliation efficiency of the bulk h-BN in different organic electrolyte solutions was dependent on the Lewis basicity of anion of the added slats. The stronger the basicity of the anion, the higher the exfoliation efficiency of the salt solution. According to the above discussion, it is understandable that when the anion of the salt has stronger Lewis basicity, the Lewis acid-base interactions between this anion and B atom (as a Lewis acid) become stronger, thus leading to the enhancement of the exfoliated efficiency and the increase in the dispersion concentration of h-BN nanosheets. This result verified the idea mentioned in the introduction. Furthermore, the difference of the exfoliation efficiency of h-BN nanosheets in sodium tartrate-NMP solution and that in potassium sodium tartrate-NMP solution indicates that the smaller size of cation of the salts can improve the exfoliation of h-BN in NMP-salt solutions. A similar conclusion has been reported in the literature. [32] Therefore, the strength of Lewis basicity of anion and size of cation of the salts play important role in the intercalation of salts into h-BN, and thus in the exfoliation of h-BN nanosheets.
Fig. 6. XRD patterns of the bulk h-BN powder and the h-BN nanosheets in NMP with and without sodium citrate.
characteristic peak at about 1371 cm−1, which is ascribed to BeN vibration mode (E2g) [28]. The positions of the E2g peaks showed slightly blue-shift (∼2 cm−1) for the exfoliated h-BN naonsheets from that of the bulk h-BN, which was consistent with the literature report that E2g peak shifted to higher frequency than that of the bulk h-BN [28]. This shift in E2g peaks can be attributed to the reduction of the h-BN layers, which leads to a higher in-plane strain and weaker interlayer interaction [29]. In addition, the intensity of E2g peak for the exfoliated h-BN nanosheets decreased dramatically compared with that for the bulk hBN, which might be ascribed to the weaker interaction between layers because of the exfoliation. The E2g peak of h-BN became broadening after exfoliation with an increase of the full width of half maximum from 7 cm−1 (bulk h-BN) to 12 cm−1 (h-BN nanosheets). This increased full width of half maximum could be attributed to the stronger surface scattering after exfoliation, which in turn influenced the vibrational excitation lifetime [30]. The Raman results further confirm that the bulk h-BN powder was exfoliated into single or few-layer h-BN nanosheets, which agrees well with the results from XRD and TEM.
4. Conclusion In summary, we developed a facile liquid phase exfoliation technique to enhance the exfoliation efficiency of few-layer h-BN nanosheets in organic electrolyte solutions. It was found that the addition of salts could effectively increase the h-BN exfoliation efficiency in NMP solvent. Among these systems, the content of h-BN dispersion could be increased up to 1.8 mg mL−1 in NMP-ethylenediaminetetraacetic acid disodium salt solution, which was about 6 times the best value reported in organic electrolyte solutions. Approximately 85% of the as-obtained flakes was 1–4 layers. Furthermore, the strength of Lewis basicity of anion and size of cation of the salts played a significant role in the exfoliation of h-BN nanosheets in NMP-salt solutions, and the salts composed of anion with stronger Lewis basicity and cation with smaller size had higher exfoliation efficiency. This method is very simple for the preparation of few-layer nanosheets of h-BN with high exfoliation efficiency and good dispersibility, which is beneficial for scale production of h-BN nanosheets.
3.3. Possible mechanism for h-BN exfoliation in NMP-salt solutions Acknowledgements In order to verify the assumption mentioned in the introduction and to understand the intercalation role of the added salts in the preparation of h-BN nanosheets, sodium citrate was selected as an example. We prepared three dispersion samples and determined the corresponding XRD spectra on three restacking films. We compared the results with those of the bulk h-BN powder and an exfoliated h-BN in NMP without sodium citrate, the result was shown in Fig. 6. It is clear that the XRD peak for (002) plane of the exfoliated h-BN in NMP without sodium citrate exhibited the same pattern as the (002) peak of the bulk h-BN powder. This suggests that the interlayer spacing of h-BN was not changed after exfoliation in NMP, thus indicating that the NMP molecules did not intercalate into the interlayer of h-BN. However, the exfoliated h-BN in NMP‑sodium citrate solution showed the XRD peaks at 2θ = 26.92° (for the peak of (002) plane) and 11.43°. The peak corresponding to the (002) plane became broadened relative to that of the bulk h-BN powder, and its position was similar to that of the bulk h-BN powder. A new peak was observed at a lower diffraction angle than the peak of (002) plane. These results indicate that the interlayer spacing was partially expanded after exfoliation in NMP‑sodium citrate solution, but the crystal structure was not destroyed, suggesting a salt intercalation mechanism for the exfoliation of h-BN nanosheets. In addition, it was found that the exfoliation efficiency of the bulk hBN in different NMP-salt solutions increased in the sequence:
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Full length article
Scalable exfoliation and dispersion of few-layer hexagonal boron nitride nanosheets in NMP-salt solutions ⁎
T
⁎
Huiyong Wang , Xin Su, Tao Song, Zhiyong Li, Yuling Zhao, Hao Lou, Jianji Wang
Henan Key Laboratory of Green Chemistry, Collaborative Innovation Center of Henan Province for Green Manufacturing of Fine Chemicals, Key Laboratory of Green Chemical Media and Reactions, Ministry of Education, School of Chemistry and Chemical Engineering, Henan Normal University, Xinxiang, Henan 453007, PR China
A R T I C LE I N FO
A B S T R A C T
Keywords: Hexagonal boron nitride Liquid-phase exfoliation NMP-salt solutions Interactions
Efficient and large-scale production of boron nitride (h-BN) nanosheets is very important for their applications. Here, we have developed a facile strategy for liquid-phase exfoliation of h-BN nanosheets in organic electrolyte solutions where N-methyl-pyrrolidinone (NMP) is used as an organic solvent, and sodium citrate, sodium tartrate, ammonium oxalate, potassium sodium tartrate or ethylenediaminetetraacetic acid disodium salt is used as a salt. It is found that the dispersion content of h-BN nanosheets has been greatly improved by using organic electrolyte solutions. For example, by addition of ethylenediaminetetraacetic acid disodium salt into NMP, the content of the exfoliated h-BN is 36 times that in neat NMP. Furthermore, approximately 85% of the h-BN nanosheets obtained in such a strategy is 1–4 layers. Mechanism study indicates that the strength of Lewis basicity of anion and size of cation of the salts play an important role in the exfoliation of h-BN by organic electrolyte solutions. The liquid exfoliation strategy reported here shows great promise for the mass production of high-quality few-layer h-BN nanosheets.
1. Introduction Two dimensional (2D) nanosheets such as graphene, hexagonal boron nitride (h-BN), and transition metal dichalcogenides have been receiving extensive attention in recent years, due to their ultrathin structure and outstanding properties [1,2]. The merits of these materials make them great potential in a huge range of applications in scientific and technological fields. Among these 2D nanosheets, the h-BN nanosheets, so called “white graphene”, has also been intensively studied in recent years because of their advantages compared to graphene. In addition to the unique chemical and thermal stability [3–5], h-BN nanosheets also show great mechanical strength and electrical insulating properties (band gap of 5–6 eV) [6,7]. Therefore, h-BN nanosheets have wide applications in sensors, energy storage devices, temperature antioxidant materials, composite materials and other fields [8]. To promote large-scale applications of h-BN nanosheets, the production of single or few layers of h-BN nanosheets (< 5 layers) in large quantity and high quality become very important. Much effort has been devoted to develop various preparation methods for h-BN nanosheets. To date, h-BN nanosheets have been prepared via chemical vapor deposition, segregation methods, and mechanical or sonication exfoliation approaches [9–15]. The chemical vapor deposition and segregation methods can provide single-layer h-BN nanosheets, but usually need ⁎
extreme conditions of temperature and pressure. On the other hand, mechanical or sonication exfoliation method is usually high energy/ time consuming. Therefore, they are not widely considered as suitable to synthesize h-BN nanosheets on larger scales. Liquid-phase exfoliation is one of the most potential methods for preparing h-BN nanosheets with high quality from bulk h-BN powder on large scales. In general, h-BN powder is mixed with a solvent, and ultrasonic energy is introduced into the system in the liquid-phase exfoliation. To obtain high yield of exfoliated h-BN nanosheets, good candidate solvents possessing the ability to minimize the enthalpy of the whole mixture and the energy for exfoliation are preferred. Various solvents have already been investigated for the dispersion and exfoliation of h-BN using sonication in the literatures [16–22], such as isopropyl alcohol, N, N-dimethylformamide, dimethyl sulfoxide, and NMP. To improve dispersion and exfoliation of h-BN, there are also some successful attempts to combine two or more solvents during sonication [23,24]. However, only 5% -10% yield could be achieved by using these solvents. In addition, compared to graphene, the partial ionic character in the BeN bond of h-BN and the “lip-lip” interactions between neighboring layers make the exfoliation of h-BN bulk more difficult [25]. Therefore, the liquid exfoliation of bulk h-BN is even more challenging than other layered materials, and more efforts should be devoted to increasing yield of the exfoliated h-BN nanosheets and
Corresponding authors. E-mail addresses: [email protected] (H. Wang), [email protected] (J. Wang).
https://doi.org/10.1016/j.apsusc.2019.05.296 Received 11 March 2019; Received in revised form 23 May 2019; Accepted 25 May 2019 Available online 27 May 2019 0169-4332/ © 2019 Elsevier B.V. All rights reserved.
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diffractometer (D8&Advance, Bruker) with monochromatic Cu Kα radiation (λ = 1.5418 Å). AFM analysis was carried out in an AFM instrument controlled with the Nanoscope V software V8 (Bruker Corporation, UK). Raman spectra were recorded on a HORIBA JobinYvon XploRA spectrophotometer with 633 nm excitation. Yield of the exfoliated h-BNNSs was calculated from the following equation:
the dispersion content. In principle, due to the electron deficient behavior of B atoms, h-BN owns Lewis acid characteristics and is vulnerable to attack from Lewis base compounds, especially at the defect sites in h-BN [25]. It is conceivable that if a Lewis base compound is added in a solvent, a Lewis acid-base complex can be formed, which may help to overcome the interaction between layers of h-BN, and consequently large amount of h-BN nanosheets would be easily exfoliated from h-BN bulk with the assistance of vigorous sonication. In addition, the presence of Lewis acid-base complexes will promote the dispersion of h-BN nanosheets and enhance the dispersion content and yield of h-BN nanosheets. In order to verify this concept in this work, we report liquid exfoliation of h-BN nanosheets in five kinds of binary organic electrolyte solutions. For this purpose, NMP is chosen due to the fact that it is the best solvent for the exfoliation of h-BN nanosheets. At the same time, five kinds of salts including sodium citrate, sodium tartrate, ammonium oxalate, potassium sodium tartrate, and ethylenediaminetetraacetic acid disodium salt are selected as intercalators to examine the relationship between the structures of salts and the exfoliation performance. The optimization of the exfoliating process is studied systematically, and hBN nanosheets have been characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM), atomic force microscopy (AFM), Raman and ultraviolet visible absorption spectroscopy. It is found that the h-BN nanosheets are predominantly present in 1–4 layers, and the addition of salts significantly increased exfoliation efficiency and dispersion content of the h-BN nanosheets. The dispersion content (up to around 1.8 mg mL−1) is 36 times that in neat NMP, and about 6 times the best value reported in organic solutions.
Yield (%) = (weight of h − BN nanosheets) × 100/(weight of h − BN bulk) (1)
3. Results and discussion 3.1. Optimization of exfoliation efficiency Here, five different salts were used to examine the effect of chemical nature of the salt on the exfoliation performance of the h-BN powder in NMP. It was found that among the salts investigated in this work, only sodium citrate, sodium tartrate, and ethylenediaminetetraacetic acid disodium salt could significantly increase the yield of h-BN nanosheets in NMP solvent. Therefore, these three kinds of salts were taken to examine salt effect in exfoliation efficiency of bulk h-BN powder. To maximize the exfoliation efficiency of h-BN powder, it is necessary to optimize the experimental conditions for the preparation of hBN nanosheets. In doing so, the effect of the starting h-BN mass, salt concentration, sonication time, and system temperature on the dispersed content and yield of h-BN nanosheets was investigated in detail. To conveniently determine the optimized conditions, the absorbance values of h-BN nanosheets dispersions were used to estimate the concentrations of h-BN dispersion solution, which is based on the LambertBeer law. Figs. S1–S4 (Supporting information) show the UV–vis data for h-BN nanosheets dispersion prepared under different experimental conditions. Because h-BN and salts do not exhibit any observable absorption peaks, a wavelength of 300 nm was used to compare the relative absorbance between samples. All data are the averaged results of three trials for each system. Taking organic electrolyte solution containing ethylenediaminetetraacetic acid disodium as an example, it was found that the optical absorbance of h-BN nanosheets in solution was almost unchanged with the variety of the system temperature from 15 to 75 °C (as shown in Fig. S1). So, 25 °C was chosen in this work. Furthermore, the optical absorbance of h-BN nanosheets in solution was firstly increased and then decreased with the increase of ethylenediaminetetraacetic acid disodium salt content. Therefore, the optimum concentration of ethylenediaminetetraacetic acid disodium salt in NMP was 20 mg mL−1. At last, the absorbance values of the h-BN dispersions increased rapidly with the increase of the starting h-BN mass and sonication time up to 12 mg mL−1 and 12 h, respectively, and then kept at an almost constant absorbance. Thus, the optimal starting h-BN mass and sonication time was 12 mg mL−1 and 15 h, respectively. The optimized conditions for the preparation of h-BN nanosheets in different organic electrolyte solutions were listed in Table 1. The content of these final h-BN nanosheets dispersions was estimated using the optical absorption spectrum and the Lambert-Beer law, which was presented in Table 1. The yield of the as-obtained h-BN nanosheets was calculated from Eq. (1), and the results were shown in Table 1. It can be seen that compared to neat NMP, the concentrations and yields of h-BN nanosheets in organic electrolyte solutions were effectively improved. The content of h-BN nanosheets dispersion prepared in NMP-ethylenediaminetetraacetic acid disodium salt (sodium citrate or sodium tartrate) solutions was 36, 22 and 11 times that in neat NMP, and the yield of hBN nanosheets was increased by 36.5, 21.5, and 10.5 times, respectively. Interestingly, the dispersion content of h-BN nanosheets in ethylenediaminetetraacetic acid-NMP solution was around 6 times that obtained in hyperbranched polyethylene + CHCl3 solution, [26] which provides the best yield for the liquid exfoliation of h-BN up to now. It
2. Experimental 2.1. Materials Hexagonal boron nitride (h-BN, 98.5%, 1 μm particle size), N-methyl-pyrrolidinone (99%), sodium citrate (98%) and sodium tartrate (99%) were purchased from Aladdin Co., Ltd. (Shanghai, China). Ammonium oxalate (99.8%), potassium sodium tartrate (99.5%), and ethylenediaminetetraacetic acid disodium salt (99%) were obtained from Shanghai Macklin biochemical Co., Ltd. These chemicals were directly used without further purification. 2.2. Methods A given quantity of the bulk h-BN powder was mixed with binary organic electrolyte solutions. Then, the mixture was added into a centrifuge tube and sonicated in a bath sonicator (KQ-400GKDV) at a power of 400 W, and the sonicator was equipped with a temperature control system to prevent overheating during sonication. After sonication, the exfoliated solution was centrifuged at 4000 rpm, relative centrifugal force (RCF) = 1644 g, for 40 min to remove the un-exfoliated h-BN. Subsequently, the top 2/3 of the dispersion was gathered carefully, centrifuged at 10000 rpm (RCF = 10,278 g) for 10 min, and the precipitate was collected. To remove the salt, the precipitate was washed by 10 mL water under sonicating for 5 min, then centrifuged at 5000 rpm (RCF = 2569 g) for 5 min, and re-gathered. The rinsing process was repeated for four times. 2.3. Characterization The concentration of h-BN nanosheets was determined by UV–Vis spectrometer (TU-1900) at 300 nm with corresponding organic electrolyte solutions as blanks. SEM image was performed on field-emission SEM system (Hitachi SU8010). The TEM image was obtained using a JEM 2100 electron microscope with the accelerating voltage of 200 kV. XRD patterns of h-BN nanosheets were acquired by X-ray powder 657
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Table 1 The optimized conditions for the preparation of h-BN nanosheets in different salt solutions of NMP together with the corresponding dispersion concentration and yield of h-BN nanosheets. Salt
Ethylenediaminetetraacetic acid disodium Sodium citrate Sodium tartrate Neat NMP
Csalt (mg mL−1)
Ch-BN (mg mL−1)
Sonication time (h)
Dispersion content (mg mL−1)
Yield (%)
20 23 10 0
12 12 12 12
15 15 15 15
1.80 1.08 0.56 0.05
15 9 4.7 0.4
Fig. 1. SEM images of the bulk h-BN powder and the h-BN nanosheets prepared in ethylenediaminetetraacetic acid disodium salt-NMP solution.
bulk h-BN powder also exhibited distinct differences. h-BN nanosheets had an ultrathin, transparent and overlapped flat structure, whereas a bulky and opaque structure was observed for the parent h-BN, which provides clear evidence that the h-BN sheets were exfoliated thoroughly into very thin layers. In addition, SAED pattern of the h-BN nanosheets prepared in NMP-salts solutions illustrated the hexagonally symmetric structure of h-BN. [27] These results suggest that the crystal structure of h-BN nanosheets was not destroyed during sonication process and their single crystalline was retained. The specific lateral dimensions and thickness of the exfoliated h-BN nanosheets were analyzed by atomic force patterns. Larger than 100 hBN nanosheets pieces were measured for each sample. The detailed statistical analysis from the AFM images of h-BN nanosheets was performed and a histogram of the thickness distribution from AFM images was plotted (Figs. 3 and S6). It was found from Fig. 3 that the thickness of approximately 85% h-BN nanosheets prepared in this work was
can be seen clearly that the dispersion content of the as-exfoliated h-BN nanosheets reported in this work is significantly higher than that reported previously. 3.2. Morphology and quality of the exfoliated h-BN nanosheets In order to illustrate the morphology of exfoliated h-BN nanosheets, scanning electron microscopy (SEM) were used. Fig. 1 shows the SEM images of the bulk h-BN powder and the exfoliated h-BN nanosheets. Compared with the bulky h-BN precursors, the h-BN nanosheets demonstrate a much smaller size and a nanosheet-like morphology. Figs. 2 and S5 exhibited typical TEM images and corresponding selected area electron diffraction (SAED) patterns of the h-BN nanosheets and parent bulk h-BN powder. It was found that the irregular shaped h-BN nanosheets with lateral sizes ranging from a few tens of nanometers to as long as over 100 nm. The TEM images of h-BN nanosheets and parent
Fig. 2. TEM image (a) and corresponding SAED pattern (b) of the h-BN nanosheets prepared in ethylenediaminetetraacetic acid disodium salt-NMP solutions. 658
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Fig. 3. AFM image and the layer number distribution of h-BN nanosheets prepared in ethylenediaminetetraacetic acid disodium salt-NMP solutions.
0.4–1.8 nm. Given that the thickness of a single-layered h-BN is about 0.4–0.5 nm, [10] this observation indicates that the as-prepared h-BN nanosheets was mostly constitutive of 1–4 atomic layers. This result suggested that using NMP-salt solution is a promising strategy for the exfoliation of few-layer h-BN nanosheets. Moreover, the nanosheets showed lateral dimensions about 100 nm, which is consistent with that observed from TEM. To further characterize the crystalline structure of the exfoliated hBN nanosheets, XRD measurements were performed, and the results for the bulk h-BN powder and the exfoliated h-BN nanosheets in different NMP-salts solutions were shown in Fig. 4. It can be seen from Fig. 4 that XRD finger print peak of the bulk h-BN powder was present at 26.75°, 41.7°, 55.3°and 75.9°, which are assigned as its (002), (004), (100), and (101) plane, respectively. This result indicates a typical hexagonal structure of h-BN (JCPDS card no. 01-073-2095). It is known that the broadness of the peak of (002) plane has been widely used for the analysis of single or fewer layered graphene. Similarly, thickness of the exfoliated h-BN nanosheets can also be determined based on the calculation of full width of half maximum of the peak of (002) plane. The
full width of half maximum of the peak of (002) plane for the exfoliated h-BN nanosheets in different NMP electrolyte solutions was broader (∼0.08°) compared to the bulk h-BN. The broadness of the diffraction peak of (002) plane signifies the nanoscale thickness nature (∼2 nm thickness calculated by Scherrer equation). This result confirms the thickness characterization of the exfoliated h-BN nanosheets observed from TEM technique. In addition, the intensity of the peak of (002) plane for h-BN nanosheets significantly decreased and two-theta of this peak slightly downshifted from 26.75 to 26.65 °C, corresponding to an increase in the interplanar distance from 0.33 nm to 0.34 nm computed from Bragger equation. The increased interplanar distance and the decreased intensity of other diffraction peaks [(100), (104) and (110)] suggest the formation of ultrathin h-BN nanosheets with a less extended/ordered stacking in the c direction. The thickness of the h-BN nanosheets and their crystalline structures information were also provided by Raman spectroscopy measurements according to the shift and broadness of the Raman peaks. As shown in Fig. 5, the Raman spectra of bulk h-BN and h-BN nanosheets exhibited a
Fig. 4. XRD patterns of the bulk h-BN powder and the as-exfoliated h-BN nanosheets in different NMP-salts solutions.
Fig. 5. Raman spectra for the bulk h-BN powder and the exfoliated h-BN nanosheets prepared in different NMP-salts solutions. 659
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ethylenediaminetetraacetic acid disodium salt > sodium citrate > sodium tartrate > ammonium oxalate, which was approximately consistent with the order of pKa values of the corresponding acid in water (6.16, 6.4, 4.37, 4.19 for ethylenediaminetetraacetic acid, citrate acid, tartrate acid and oxalate acid, respectively). [31] This indicates that the exfoliation efficiency of the bulk h-BN in different organic electrolyte solutions was dependent on the Lewis basicity of anion of the added slats. The stronger the basicity of the anion, the higher the exfoliation efficiency of the salt solution. According to the above discussion, it is understandable that when the anion of the salt has stronger Lewis basicity, the Lewis acid-base interactions between this anion and B atom (as a Lewis acid) become stronger, thus leading to the enhancement of the exfoliated efficiency and the increase in the dispersion concentration of h-BN nanosheets. This result verified the idea mentioned in the introduction. Furthermore, the difference of the exfoliation efficiency of h-BN nanosheets in sodium tartrate-NMP solution and that in potassium sodium tartrate-NMP solution indicates that the smaller size of cation of the salts can improve the exfoliation of h-BN in NMP-salt solutions. A similar conclusion has been reported in the literature. [32] Therefore, the strength of Lewis basicity of anion and size of cation of the salts play important role in the intercalation of salts into h-BN, and thus in the exfoliation of h-BN nanosheets.
Fig. 6. XRD patterns of the bulk h-BN powder and the h-BN nanosheets in NMP with and without sodium citrate.
characteristic peak at about 1371 cm−1, which is ascribed to BeN vibration mode (E2g) [28]. The positions of the E2g peaks showed slightly blue-shift (∼2 cm−1) for the exfoliated h-BN naonsheets from that of the bulk h-BN, which was consistent with the literature report that E2g peak shifted to higher frequency than that of the bulk h-BN [28]. This shift in E2g peaks can be attributed to the reduction of the h-BN layers, which leads to a higher in-plane strain and weaker interlayer interaction [29]. In addition, the intensity of E2g peak for the exfoliated h-BN nanosheets decreased dramatically compared with that for the bulk hBN, which might be ascribed to the weaker interaction between layers because of the exfoliation. The E2g peak of h-BN became broadening after exfoliation with an increase of the full width of half maximum from 7 cm−1 (bulk h-BN) to 12 cm−1 (h-BN nanosheets). This increased full width of half maximum could be attributed to the stronger surface scattering after exfoliation, which in turn influenced the vibrational excitation lifetime [30]. The Raman results further confirm that the bulk h-BN powder was exfoliated into single or few-layer h-BN nanosheets, which agrees well with the results from XRD and TEM.
4. Conclusion In summary, we developed a facile liquid phase exfoliation technique to enhance the exfoliation efficiency of few-layer h-BN nanosheets in organic electrolyte solutions. It was found that the addition of salts could effectively increase the h-BN exfoliation efficiency in NMP solvent. Among these systems, the content of h-BN dispersion could be increased up to 1.8 mg mL−1 in NMP-ethylenediaminetetraacetic acid disodium salt solution, which was about 6 times the best value reported in organic electrolyte solutions. Approximately 85% of the as-obtained flakes was 1–4 layers. Furthermore, the strength of Lewis basicity of anion and size of cation of the salts played a significant role in the exfoliation of h-BN nanosheets in NMP-salt solutions, and the salts composed of anion with stronger Lewis basicity and cation with smaller size had higher exfoliation efficiency. This method is very simple for the preparation of few-layer nanosheets of h-BN with high exfoliation efficiency and good dispersibility, which is beneficial for scale production of h-BN nanosheets.
3.3. Possible mechanism for h-BN exfoliation in NMP-salt solutions Acknowledgements In order to verify the assumption mentioned in the introduction and to understand the intercalation role of the added salts in the preparation of h-BN nanosheets, sodium citrate was selected as an example. We prepared three dispersion samples and determined the corresponding XRD spectra on three restacking films. We compared the results with those of the bulk h-BN powder and an exfoliated h-BN in NMP without sodium citrate, the result was shown in Fig. 6. It is clear that the XRD peak for (002) plane of the exfoliated h-BN in NMP without sodium citrate exhibited the same pattern as the (002) peak of the bulk h-BN powder. This suggests that the interlayer spacing of h-BN was not changed after exfoliation in NMP, thus indicating that the NMP molecules did not intercalate into the interlayer of h-BN. However, the exfoliated h-BN in NMP‑sodium citrate solution showed the XRD peaks at 2θ = 26.92° (for the peak of (002) plane) and 11.43°. The peak corresponding to the (002) plane became broadened relative to that of the bulk h-BN powder, and its position was similar to that of the bulk h-BN powder. A new peak was observed at a lower diffraction angle than the peak of (002) plane. These results indicate that the interlayer spacing was partially expanded after exfoliation in NMP‑sodium citrate solution, but the crystal structure was not destroyed, suggesting a salt intercalation mechanism for the exfoliation of h-BN nanosheets. In addition, it was found that the exfoliation efficiency of the bulk hBN in different NMP-salt solutions increased in the sequence:
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