Facile and scalable preparation of ultra-large boron nitride nanosheets and their use for highly thermally conductive polymer composites

Abstract

Due to their excellent physical and chemical properties, boron nitride nanosheets (BNNSs) have shown great application potential in many fields. However, the difficulty in scalable preparation of large-size BNNSs is still the current factor that limits this. Herein, a simple yet efficient microwave-assisted chemical exfoliation strategy is proposed to realize scalable preparation of BNNSs by using perchloric acid as the edge modifier and intercalation agent of h-BN. The as-obtained BNNSs behave a thin-layered structure (average thickness of 3.9 nm) with a high yield of ~16%. Noteworthy, the size of BNNSs is maintained to the greatest extent so as to realize the preparation of BNNSs with ultra-large size (up to 7.1 μm), which is the largest so far obtained for the top-down exfoliated BNNSs. Benefiting from the large size, it can significantly improve the thermal diffusion coefficient and the thermal conductivity of polyvinyl alcohol by 51 and 62 times respectively, both showing a higher value than the one previously reported. This demonstrates that the prepared BNNSs have great promise in enhancing the thermal conductivity of polymer materials.

Graphical abstract

Keywords

boron nitride nanosheet / thermal conductivity / chemical exfoliation / large size

Contents

Introduction

Experimental

Preparation of BNNSs

Preparation of PVA/BNNS composite films

Characterization

Results and discussion

Conclusions

Disclosure of potential conflicts of interest

Acknowledgements

References

Supplementary information

1 Introduction

The use of miniaturized, highly integrated and high-power density electronic devices will result in the generation of a large amount of heat, and the continuous accumulation of heat will seriously affect their performance and service life [1–3]. Therefore, this puts forward higher requirements for the development of high-performance thermal management materials. Among them, polymer-based composite materials have been widely used in the field of microelectronic packaging due to their low cost, processability, electrical insulation, light weight, flexibility, etc. [4–5]. However, the thermal conductivity of polymers in general is not ideal (about 0.2 W·m⁻¹·K⁻¹), so fillers with high thermal conductivity such as graphene, aluminum nitride, carbon nanotubes, and boron nitride (BN) need to be added to enhance its thermal conductivity [6–9].

Hexagonal boron nitride (h-BN), a similar structure to graphite, has been widely used as the attractive thermally conductive filler due to its high thermal conductivity, electrical insulation, and corrosion resistance [10–12]. Compared with bulk h-BN (400 W·m⁻¹·K⁻¹), boron nitride nanosheets (BNNSs) with a two-dimensional (2D) structure show higher thermal conductivity (600 W·m⁻¹·K⁻¹) due to the reduction of phonon‒phonon scattering with decreasing the number of layers [13], and the thin-layer structure of BNNSs is more conducive to the formation of heat conduction network in the polymer [14–15]. In addition, as an important parameter to improve the thermal conductivity of polymers, the lateral size of BNNSs also plays a vital role [13]. BNNSs with the small lateral size will usually significantly increase the interfacial thermal resistance, resulting in a lower thermal conductivity of the composites [16–17]. In general, the realization of the scalable preparation of large-size BNNSs is particularly important to better meet its application requirement.

Up to now, many strategies have been developed for the preparation of BNNSs. Generally speaking, these preparation strategies can be classified into two major categories according to their production mechanisms: the bottom-up route and the top-down route [18]. The former mainly relies on the high-temperature treatment of various precursors on substrates to form few-layered BNNSs [19]. However, some challenging issues, including high cost and complex hardware systems, limit their practical applications. The latter is mainly based on the exfoliation of bulk h-BN, which is performed via utilizing additional mechanical or chemical force to the h-BN layer to overcome the van der Waals forces [20–21]. However, the higher exfoliation energy of h-BN, which is ~33% higher than that of graphite due to the smaller distance and the ‘lip‒lip’ interactions between h-BN layers [22], directly makes it more difficult to be exfoliated into BNNSs with large lateral size. For example, as a commonly-used strategy for the exfoliation of h-BN, liquid-phase exfoliation of h-BN to few-layer BNNSs can be realized with the assistance of the mechanical external force generated from an ultrasonicator. However, usually the lateral size of BNNSs obtained is very small (mostly≤1 μm), since the h-BN sheets can be easily broken up into little fragments under ultrasonic treatment [23–24]. Ball-milling can also exfoliate the bulk h-BN into few-layered BNNSs. Nevertheless, the violent collision between milling balls and bulk h-BN results in a significant decrease in the lateral size of the as-prepared BNNSs, which ranges from tens to hundreds of nanometers only [25–26]. By comparison, the chemical treatment seems more advantageous in the preparation of larger-size BNNSs, since the chemical reagents can modify the edge structure of h-BN or expand its interlayer spacing by introducing foreign substances, which is beneficial to further realize the effective exfoliation of h-BN [27–28]. For example, Yuan et al. introduced hydroxyl functional groups at the edge of h-BN by potassium hydroxide pretreatment and further expanded the interlayer spacing of h-BN by means of low-temperature hydrogen annealing. As a result, the average lateral size of the obtained BNNS by exfoliating the treated h-BN can reach 1.6 μm [17]. Also, a modified Hummers’ chemical treatment route for the exfoliation of h-BN was reported by Du et al. [29]. Although the resultant lateral size of the BNNSs is up to micron level (~3 μm), the long reaction period, pollution of heavy metal ions (e.g., Mn²⁺) and complicated preparation process will greatly limit its practical application. Thus, the facile and scalable methods for the high efficiency production of BNNSs with large lateral size are still urgently desired.

Herein, we present a facile and cost-effective microwave-assisted chemical method to produce few-layer BNNSs with large lateral size. In this method, the bulk h-BN is soaked in perchloric acid (HClO₄) to modify the edge of h-BN and insert into the h-BN layer. Then, a short-time microwave treatment on the soaked h-BN leads the exfoliation of h-BN to few-layer BNNSs. Interestingly, the obtained BNNSs have an ultra-large lateral size (the maximum lateral size can be up to 16 μm, and the average sheet lateral size is 7.1 μm). When added into polyvinyl alcohol (PVA), BNNSs can greatly improve the thermal conductivity of PVA, demonstrating that the prepared large-size BNNSs have a great application potential for improving the thermal conductivity of polymer materials.

2 Experimental

2.1 Preparation of BNNSs

Firstly, 0.4 g h-BN powder (mean particle size of about 15 μm) and 10 mL HClO₄ (72%) were completely mixed in a 50 mL flask by stirring for 30 min at room temperature. Subsequently, the exfoliation of h-BN would be carried out in two steps, including a pre-soaking step and a microwave-heating step. In the pre-soaking step, the flask was transferred into an oil bath at 180 °C and stayed for 10 h under magnetic stirring. At this time, the obtained BN product was labeled BN-180. In the microwave (MW)-heating step, the mixed solution of HClO₄ containing BN-180 was transferred into a quartz beaker, and then immediately MW-irradiated for ~6 min with a 700 W ordinary household MW oven (provided by Media, China). During the MW irradiation period, a large amount of gases would be generated, resulting in the exfoliation of h-BN (denoted as exf-BN).

In order to separate the obtained BNNSs and calculate its exfoliation yield, 0.1 g exf-BN was added into 100 mL isopropanol (IPA) and sonicated for 30 min to obtain a uniform dispersion. The dispersion was centrifuged at a speed of 600 r·min⁻¹ for 10 min, then the supernatant was poured out carefully, and an appropriate amount of IPA was continued to be added into the precipitate, repeating the ultrasonic dispersion and centrifugation process until the supernatant was colorless. Among them, the product in the supernatant portion (denoted as BNNSs) was used for further characterization. The unexfoliated h-BN (mark its mass as m₁, unit: g) at the bottom was dried at 60 °C for 24 h, which is used to calculate the exfoliation yield (η):

(1)\eta /\%={\displaystyle \frac{\mathrm{0.1}-(m_{\mathrm{1}}/\mathrm{g})}{\mathrm{0.1}}}\times \mathrm{100}

Moreover, in order to compare the products obtained under different reaction conditions, samples that were pre-soaked in HClO₄ for 0, 5 and 15 h were also prepared and labeled as BNNS-0, BNNS-5 and BNNS-15, respectively, with other conditions remaining unchanged. Meanwhile, samples obtained by the pre-soaking process in HClO₄ for 10 h as well as the MW-heating treatment for only 3 min were labeled as BNNS-3MW.

2.2 Preparation of PVA/BNNS composite films

Typically, a 5 wt.% PVA solution was firstly prepared by dissolving PVA in deionized water under magnetic stirring at 90 °C for 2 h. The BNNS dispersion (1 mg·mL⁻¹) was prepared by adding BNNS powders in the IPA solution, followed by the sonication for 0.5 h. Subsequently, the dispersion was vacuum-filtered to make BNNSs uniformly deposited on the poly(tetrafluoroethylene) (PTFE) membrane (47 mm in diameter, 0.22 μm in pore size) to form the disk-shape BNNS specimen. Then, 3 mL of the PVA solution was gently added onto the BNNS specimen and dried in an oven at 60 °C for 8 h. Finally, the PVA/BNNS composite film was peeled off directly from the membrane filter. To investigate the effect of the BNNS content, different PVA/BNNS composite films with the BNNS contents of 0, 15, 25, 35, and 45 wt.% were prepared, denoted as PVA/BNNS-0, PVA/BNNS-15, PVA/BNNS-25, PVA/BNNS-35, and PVA/BNNS-45, respectively. For comparison, PVA/h-BN-35 composite films with 35 wt.% h-BN as the filler were also prepared by using the same film-making method.

2.3 Characterization

Fourier transform infrared spectroscopy (FTIR) was performed by a Fourier transform infrared spectrometer (E55Xfra106, BRUKER, Germany). X-ray diffraction (XRD) study was carried out at a Rigaku D/MAX-2500 powder diffractometer with Cu Kα radiation (λ = 0.154 nm) operated at 40 kV and 200 mA, and Raman spectra were recorded via a Renishaw inVia Raman microscope with a laser wavelength of 532 nm. X-ray photoelectron spectroscopy (XPS) was performed by using a K-alpha spectrometer (Thermo VG Corporation, USA) with the Al Ka radiation (1486.6 eV, 15 kV). The morphology and the microstructure of h-BN, BN-180 and BNNSs were characterized by field-emission scanning electron microscopy (FESEM; Hitachi S-4800, operated at 15 kV), transmission electron microscopy (TEM; JEOL JEM-2010, operated at 200 kV), and atomic force microscopy (AFM; Bruker-Multi Mode 8). Tensile tests were performed on a universal tester (CMT6103, China) with a strain rate of 4 mm·min⁻¹. The thermal diffusivity (α) values of PVA/BNNS composite films were detected by using a laser-flash diffusivity instrument LFA 467 (NETZSCH, Germany). The density (ρ) values of composite films were obtained by using a micrometer for dimensional measurements and a microbalance for weight measurements. The specific heat capacity (c) values of composite films were detected from differential scanning calorimetry (DSC; 200 F3, NETZSCH, Germany). Both the thermal diffusivity and the specific heat capacity were characterized at room temperature. The thermal conductivity (κ) was calculated from the equation: κ = α×ρ×c.

3 Results and discussion

Due to the shorter interlayer distance and the partial ionic bonding between h-BN layers (i.e., lip‒lip interactions), the exfoliation of h-BN is much more difficult than that of graphite [30]. Herein, to facilitate the exfoliation of h-BN, a novel strategy for chemically modifying the pristine h-BN is proposed, which is realized by soaking pristine h-BN in the HClO₄ solution for 10 h at a temperature of 180 °C. Figure 1(a) shows the FTIR spectrum of the HClO₄-soaked h-BN sample (BN-180). It can be seen that, besides an unimpressive peak near 3422 cm⁻¹ which is due to the absorption of trace amounts of water on the surface of h-BN [31], both pristine h-BN and BN-180 exhibit two obvious absorption peaks at 1377 and 817 cm⁻¹, which belong to the in-plane B−N stretching vibration and the out-of-plane B−N−B bending vibration, respectively [32–33]. Distinctly different from that of h-BN, however, a visible peak occurs at 1049 cm⁻¹ on the FTIR spectrum of the BN-180 sample, which is assigned to the symmetric B−O stretching (v₁) band, demonstrating that BN-180 has been functionalized with oxygen functional groups [34]. Figure 1(b) displays XRD patterns of h-BN and BN-180 samples. It is seen that both of them exist five similar typical peaks at 26.6°, 41.4°, 43.8°, 50.0° and 53.18°, associated to (0 0 2), (1 0 0), (1 0 1), (1 0 2) and (0 0 4) planes of h-BN, respectively. It is worth noting that the (0 0 2) peak position of BN-180 shifts from 26.66° to 26.52°, indicating an increase in the interplanar distance [d_{(0 0 2)}] from 3.34 to 3.36 Å [35–36]. The d_{(0 0 2)} increment (Δd_{(0 0 2)}) for BN-180 can be deduced to be 0.002 nm, which should be ascribed to the intercalation of HClO₄ between interlayers of h-BN during the long-term soaking process. This enlarged interlayer distance can also be clearly observed (Figs. 1(c) and 1(d)). So, we can conclude that the HClO₄-soaking can effectively expand the interlayer spacing of h-BN, which will facilitate its exfoliation into BNNS under the assistance of the MW irradiation.

Next, the BN-180 sample was soaked again in the HClO₄ solution at room temperature followed by the MW-irradiation and then the sonication, thus acquiring the exfoliated BNNS sample. Figure 2 shows typical morphological characteristics of the resultant BNNS sample. It can be clearly seen that the obtained BNNSs exhibit a wrinkled and semi-transparent film-like structure, demonstrating that BNNSs with the thin-layer structure have been successfully exfoliated from bulk h-BN [27].

Based on above results, the possible mechanism for the chemical exfoliation of h-BN may be deduced as follows: when pristine h-BN is soaked in the HClO₄ solution, the strong oxidizing property of HClO₄ will enable the edges of h-BN to be oxidized (demonstrated by the FTIR characterization in Fig. 1(a)), resulting in the opening of h-BN edges. This edge-opening will facilitate HClO₄ to intercalate into interlayers between adjacent h-BN nanosheets, leading to a larger interlayer spacing of h-BN (as shown in Figs. 1(b) and 1(d)). When the soaked h-BN is soaked again in the HClO₄ solution, the larger interlayer spacing will allow more HClO₄ to intercalate among h-BN interlayer galleries, and the subsequent MW-irradiation will result in the rapid decomposition of introduced HClO₄ into gaseous substances (e.g., H₂O, O₂ and Cl₂), as shown in Eq. (1). Once the instantaneous gas pressure rise within the h-BN interlayer gallery can overcome the van der Waals force, the soaked h-BN will be effectively exfoliated into BNNSs. Figure 3 provides a schematic illustration of the fabrication mechanism of BNNSs. In addition, the analysis of products obtained under different reaction conditions (such as pre-soaking time and MW conditions) also proves that the proper pre-soaking time and sufficient gasification of HClO₄ under MW conditions are the basis for obtaining thin-layer structure BNNSs. This further validates the mechanism of the h-BN exfoliation proposed above from the side (Fig. S1, see the section on Supplementary Information for detailed experimental description):

(2)\mathrm{4}{\mathrm{HClO}}_{\mathrm{4}}\stackrel{\mathrm{\Delta }}{\to }\mathrm{2}{\mathrm{H}}_{\mathrm{2}}\mathrm{O}\uparrow +\mathrm{7}{\mathrm{O}}_{\mathrm{2}}\uparrow +\mathrm{2}{\mathrm{Cl}}_{\mathrm{2}}\uparrow

To further evaluate the as-obtained BNNS sample, the XRD characterization was performed, as shown in Fig. 4(a). Compared with that of pristine h-BN, the (0 0 2) peak of BNNSs shows a remarkably reduced intensity, implying its few-layer feature and significantly weaker stacking in the c direction [36–37]. Figure 4(b) further provides the Raman spectrum of the as-obtained BNNS sample. Clearly, the intensity of the G band of BNNSs is apparently weaker than that of bulk h-BN, and furthermore, the G band position of BNNSs shifts to 1366.8 cm⁻¹ compared to that of h-BN (1365.0 cm⁻¹). These changes may suggest the reduction of h-BN layers [38–39]. To understand elemental compositions and chemical states of as-obtained BNNSs, the XPS measurement was conducted, as displayed in Figs. 4(c)-4(e). Chemical compositions of both BNNSs and h-BN are composed of B, N, O and C elements. For the h-BN sample, both B 1s and N 1s spectra can primarily be divided into a single peak centered at 190.5 and 398.1 eV, corresponding to the B−N bond and the N−B bond, respectively. As for the weak O 1s peak, it can be ascribed to the physically adsorbing moisture on the h-BN surface [40]. Different from those of h-BN, however, the high-resolution B 1s spectrum of BNNSs can be divided into two peaks located at 190.5 and 191.6 eV, corresponding to the B−N bond and the B−O bond, respectively. Similarly, the N 1s spectrum of BNNSs can be divided into two peaks located at 398.1 and 399.4 eV, which correspond to the N−B bond and the N−O bond, respectively [26,36,41]. The existence of B−O and N−O peaks demonstrates the introduction of oxygen into BNNSs during the MW-assisted chemical exfoliation process of h-BN.

Interestingly, no element Cl peak occurs on the XRS spectrum of the BNNS sample, indicating that HClO₄ has been thoroughly decomposed and run away in a gaseous state from the BNNS sheets during microwave irradiation process. This non-existence of residual Cl on the BNNS sheets suggests that no water-washing procedure is needed, which will save a lot of water sources. Furthermore, the gaseous substances (e.g., Cl₂) resulting from the decomposition of HClO₄ can be collected for the production of sodium hypochlorite, aluminum trichloride, iron trichloride, etc. Therefore, the high-efficiency and less environmental impact will enable the present chemical exfoliation strategy to have more advantages than those conventional chemical exfoliation methods.

Figure 5(a) displays the typical TEM image of the BNNS sample. It can be clearly seen that the as-obtained BNNSs are highly transparent to the electron beam, indicating the ultrathin structure [42]. The high-resolution TEM (HR-TEM) image (Fig. 5(b)) further proves the existence of BNNS with a single layer, and the corresponding fast Fourier transformation (FFT) demonstrates the structural ordering of BNNSs, which suggests that there is no obvious damage to the structure of BNNSs during the chemical exfoliation process with HClO₄ [43]. To measure the lateral size and determine the size distribution of BNNSs, we deposited the as-obtained BNNS dispersion on the micro-grid for SEM observation, as shown in Fig. 5(c). Figure 5(d) illustrates the lateral size distribution of the obtained BNNS sample, which is inferred by randomly measuring the lateral size of 40 BNNS sheets. The calculated average lateral size of BNNS sheets is 7.1 μm, in which about 75% of sheets are more than 4 μm with a maximum lateral size of 16 μm. As far as we know, this average lateral size is much larger than those of 2D BNNSs obtained by using the top-down exfoliation methods reported previously (Table S1). In addition, the thickness of the obtained BNNSs is also evaluated by AFM. As shown in Figs. 5(e) and 5(f), the thickness of BNNSs is no more than 7 nm with an average thickness of 3.9 nm.

Large-size BNNSs prepared by this MW-assisted chemical exfoliation method are remarkably promising in various applications, especially as thermal conducting fillers to improve the thermal conductivity of polymer materials. To verify this, different PVA/BNNS composite films with BNNS contents of 0, 15, 25, 35, and 45 wt.% (denoted as PVA/BNNS-0, PVA/BNNS-15, PVA/BNNS-25, PVA/BNNS-35, and PVA/BNNS-45, respectively) were fabricated by a vacuum filtration method. Firstly, to understand the microstructure of the composite film, SEM observation was performed on a typical PVA/BNNS composite film with a BNNS content of 35 wt.%. As displayed in Fig. 6(a), obviously, the composite film is composed of two parts. One part is an adhesive layer with PVA as the main component. The other part is the BNNS sample infiltrated with PVA, which has a densely arranged microstructure. From this, the preparation mechanism of the composite film can be deduced. Firstly, the 2D BNNS layers are easily aligned and closely lapped during the vacuum filtration process, thus forming a complete BNNS network, which is conducive to the construction of effective thermal conductivity paths. Then, with the addition of PVA, it can continuously penetrate between BNNS samples, but since the amount of PVA added is larger than the void volume between BNNS samples, a bonding layer is formed by the remaining PVA, which ensures the integrity of the composites. Moreover, the tensile stress‒strain test was performed to understand mechanical properties of the composite films. Figure 6(b) shows the tensile stress–strain curves of PVA/BNNS composite films at various BNNS loadings. The tensile strength of PVA/BNNS-0 is 90.80 MPa with an elongation at break of 14.46%. After the embedment of BNNSs, both the tensile strength and the elongation at break of the films decreased with the filler content increase. When the BNNS loading reached 45 wt.%, these mechanical properties show a more significant drop compared with others. Detailed tensile properties can be found in Table S2. This decrease in mechanical properties may be due to the aggregation of BNNSs and the poor wettability between BNNSs and PVA. However, although the addition of BNNSs resulted in a decrease in mechanical properties of PVA, the PVA/BNNS composite films still exhibit excellent flexibility. As shown in Fig. 6(c), even with 35 wt.% BNNSs, the PVA/BNNS composite film obtained in this work can be repeatedly bent at a large bending angle of 180° for 100 times without causing any visible damage.

More importantly, it is advantageous for improving the thermal conductivity of the composites at which the BNNSs with large size and thin-layered structure can be easily prone to the orientation alignment with a low interfacial thermal resistance. Figure 6(d) displays the in-plane thermal diffusivity and the thermal conductivity of all composite films. The detailed properties (κ, α, ρ, and c) can be found in Table S3. Obviously, both the thermal diffusion coefficient and the thermal conductivity become higher with the increase of the BNNS fraction. The thermal diffusion coefficient and the thermal conductivity of pure PVA are 0.133 mm²·s⁻¹ and 0.200 W·m⁻¹·K⁻¹, respectively. At the BNNS loading of 35 wt.%, the thermal diffusion coefficient and the thermal conductivity of the composite film increase to 6.813 mm²·s⁻¹ and 12.416 W·m⁻¹·K⁻¹, which are 51 and 62 times higher than those of the pure PVA film, respectively. Furthermore, the resultant in-plane thermal conductivity of PVA/BNNS-45 is 13.082 W·m⁻¹·K⁻¹, slightly higher than that of PVA/BNNS-35. Since PVA/BNNS-35 behaves a slightly lower thermal conductivity but obviously a higher tensile strength, it should be more competent than the composites with over 35 wt.% filler content (e.g., PVA/BNNS-45) for the practical applications. The through-plane thermal conductivity of PVA/BNNS-35 was also measured, which is 0.933 W·m⁻¹·K⁻¹, much lower than that of the in-plane thermal conductivity of the film, indicating its good anisotropy. Finally, a comparative composite film with h-BN (the average thickness of 1 μm) as the filler (denoted as PVA/h-BN-35) was prepared by the same method to understand how the reduced thickness of BNNSs affects the thermal conductivity of the resultant composite. As shown in Fig. S2, the in-plane thermal conductivity of PVA/h-BN-35 is significantly lower than that of PVA/BNNS-35, suggesting that the reduced thickness of BNNSs is beneficial to the in-plane thermal conductivity of the resultant composite film. This may be due to the more consistent alignment orientation of thin-layer BNNSs during the compounding process compared to h-BN, which can facilitate the construction of a more complete in-plane heat conduction path to avoid the phonon scattering [15]. Figure 6(e) summarizes the thermal conductivity of BN-based polymer composites reported previously. Clearly, the PVA/BNNS composite film prepared in this work exhibits the highest thermal conductivity [11,17,25,44–50], demonstrating that the large-size BNNSs obtained by the present strategy is very promising for the improvement of the thermal conductivity of polymer materials.

4 Conclusions

In summary, a facile yet efficient MW-assisted chemical method has been proposed for scalable preparation of BNNSs with the large-size and thin-layer structure. In this method, HClO₄ is used as the only reagent to oxidize edges of h-BN and intercalate among interlayer galleries to weaken the interlayer interaction force of h-BN. The subsequent MW irradiation leads to the rapid decomposition of the intercalated HClO₄ and thereby the effective exfoliation of h-BN. The average thickness of BNNSs obtained by this method is 3.9 nm, behaving a thin-layered structure. More interestingly, the average size of resultant BNNSs is up to 7.1 μm, which is, to the best of our knowledge, among the highest values so far reported for top-down exfoliated BNNSs. Benefiting from the large size, the prepared BNNSs have great potential in improving the thermal conductivity of polymer materials. When BNNSs as the thermal conducting filler are composited with PVA, the as-prepared PVA/BNNS composite exhibits a dramatic enhancement in the thermal conductivity compared with pure PVA. Moreover, the high efficiency and less environmental impact will enable the present chemical exfoliation strategy more competent in the scalable preparation of large-size BNNSs than those conventional chemical exfoliation methods.

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