Efficient dispersant-free liquid exfoliation down to the graphene-like state of solvent-free mechanochemically delaminated bulk hexagonal boron nitride

Abstract

It is shown that during mechanochemical treatment of the bulk hexagonal boron nitride (hBN) in the presence of an inert delamination agent the nanostructuring of hBN occurs: its partial delamination, shift of the layers relative to one another, reduction of the particle size. It is established that the reactive paramagnetic defects are formed in the structure of the nanostructured hBN, and their number is significantly reduced under the effect of water. The formation of oxygen-containing functional groups at the outer edge of the nanoparticles, and ability of such nanoparticles to effectively exfoliate in various organic solvents and in water under the effect of ultrasound are shown. The data of electron diffraction, XPS and UV-vis indicated the preservation of the hBN structure in the plane of the nanoparticles. The predominately monolayer morphology of the particles in the dispersions is confirmed by AFM microscopy and Raman spectroscopy.

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1. Introduction

Hexagonal boron nitride (hBN), as is known, possesses a structure similar to graphite, and, in the two-dimensional (2D) state, to graphene.^1,2 At the same time, a chemical composition of this material is responsible for the presence of electronic and other properties of 2D-BN, which cause it to differ significantly from graphene. So, if the graphene is a semimetal with a zero band gap, the 2D-BN is characterized by an ∼6 eV forbidden band and thus exhibits dielectric properties.^1,2 This, as well as other functional characteristics of 2D-BN, makes it a promising material for use in electronic devices based on graphene and its inorganic analogues.^2–11 Thanks to high mechanical and thermal properties, the 2D-BN could be used as a filler in polymers to increase their strength, heat resistance and thermal conductivity.^12,13 It was shown recently that the 2D-BN, in contrast to graphene, exhibits low cytotoxicity, and could be used in biosensors, for targeted delivery of drugs into the cells and their visualization.¹⁴

Chemical vapor deposition using a mixture of gases (NH₃–BH₃,^15,16 BF₃–N₂–H₂ ¹⁷etc.) is currently one of the most common methods for synthesizing 2D-BN. This method allows BN sheets of high area to be obtained. But the problems associated with the preparation of a monolayer structure, the occurrence of allotropic modifications (amorphous and cubic BN), and the appearance of impurities in the composition, are still unresolved. Besides, the conditions of such processes often require the use of toxic reagents as well as high temperatures and pressures.

The methods of producing 2D-BN directly from the bulk hBN by liquid-phase exfoliation^18,19 allow one to avoid these shortcomings. However, their application requires a prolonged ultrasound (US) treatment, and the average number of layers in the resulting nanoparticles of hBN is more than three.¹⁸ The preliminary treatment of the bulk hBN in a planetary ball mill, which is performed in the presence of various solvents, in particular benzyl benzoate which contributes to the isolation of the delaminated material and also improves the stability of the resulting 2D-BN dispersions based thereon, allows reducing the duration of the necessary US treatment.^20,21 However, the typical thickness of the particles in the dispersion is ∼2.3 nm within such an approach.

Recently, we showed on example of graphite and 2H–MoS₂ (ref. 22–26) that the dispersions of graphene and graphene oxides with different oxidation state, as well as 1H–MoS₂ with high content of monolayer particles could be prepared quite effectively by mechanochemical treatment of the corresponding bulk layered material in the presence of an inert solid delaminating agent with subsequent liquid phase exfoliation in various solvents without the use of surfactants or other auxiliary substances. We use highly water-soluble inorganic salts (NaCl in particular) as the delaminating agents that facilitate the process of their removal. We hypothesized that this method could also be applied to the hBN. In this context, the present work is devoted to preparation of graphene-like hBN (gBN) using liquid-phase exfoliation of the hBN preliminary mechanochemically delaminated without the use of solvents. Considerable attention is paid to the morphology and electronic structure of the gBN nanoparticles in the prepared dispersions as well as analyzing the possibility of functionalization of the outer boundary of the mechanochemically nanostructured hBN by the action of water to impart hydrophilicity to the resulting material, allowing its use for biological purpose, in particular.

When this work was completed, the ref. 27 devoted to the attempt of using urea and NaCl as the exfoliating agent at mechanochemical delamination of the bulky hBN appeared in the literature. But the authors of²⁷ did not succeed in preparation of the stable aqueous dispersions of hBN in the case of using NaCl probably due to different (in comparison with our work) conditions of mechanochemical treatment. The formation of defects in hBN as a result of mechanochemical processing in the inert atmosphere by ESR and results of their subsequent reaction with water were not considered in.²⁷

2. Experimental section

2.1. Materials

hBN (99.5%, #11078, Alfa Aesar) and NaCl (≥99.5%, #S7653, Sigma-Aldrich) were used as received. Purification of dimethyl formamide (DMF) (Sigma-Aldrich) before use and preparation of absolute ethanol (EtOH) (Khimlaborreaktiv) were carried out as in.^22,26 Double distilled water was used throughout the experiments.

2.2. Synthesis

A dry mixture of 50 mg of hBN and 2 g of NaCl was mechanochemically treated under argon in the agate grinding bowl of a planetary ball mill Pulverisette 6 (Fritsch) at a rotational rate of 500 rpm for 1 h. The weight ratio of the reaction mixture to milling balls was 1 : 7. The delaminating agent was removed by washing with water and the product – nanostructured hBN (nhBN) – was dried in vacuum at 60 °C. The yield of nhBN was 40 mg. Liquid-phase exfoliation of nhBN was performed in DMF, EtOH or water using US power of 20 W for 1 hour to disperse 10 mg of nhBN powder in 10 mL of the solvent. The dispersions were purified from large particles by centrifugation at 2000 rpm for 1.5 h using 5430 (Eppendorf) centrifuge and the upper half of their volume was utilized for further studies of gBN. A series of successive US disintegration/centrifugation iterations was used to determine the maximum yield of gBN relatively to the starting bulk hBN by weight method. The aqueous dispersions with higher concentration of gBN were also prepared by the analogous procedure using the initial loading of nhBN equal to 2 and 5 mg mL⁻¹.

2.3. Characterization

Powder X-ray diffraction patterns were measured with a D8 ADVANCE (Bruker) diffractometer using filtered Cu Kα radiation in the range 2θ = 2–70° with an increment of 0.05°. TEM images were obtained on a TEM125K (SELMI) microscope working at 100 kV. A copper grid covered with amorphous carbon film was used as a carrier for the samples. SAED (selected area electron diffraction) patterns were also measured using this instrument. Confocal imaging of the sample on the gold covered glass slide was done by a LSM-510 Meta confocal laser scanning microscope (Carl Zeiss) with Plan-Achromat 63x/1.4 Oil DIC oil immersion objective in Multi Track mode and excitation of fluorescence by HBO 100 ultraviolet lamp with blue light filter (FSet01 wf). AFM images were obtained using the Solver Pro M facility (NT-MDT) in a tapping mode. The samples were deposited on freshly cleaved mica (V-1 Grade, SPI Supplies) from the diluted dispersions. UV-vis spectra were measured on a double beam spectrophotometer 4802 (UNICO) with a resolution of 1 nm. Raman spectra were obtained using an inVia Raman microscope (RENISHAW) equipped with an excitation laser of 632.8 nm line in an ambient air environment. ESR and XPS spectra were registered by ELEXSYS E500 (Bruker) at the frequency of 9.861302 GHz and XPS-9200 (JEOL) with aluminum anode (10 kV, 15 mA) correspondingly.

3. Results and discussion

It was established by TEM studies, the results of which are shown in Fig. 1, that during the mechanochemical treatment of the bulk hBN in the presence of NaCl its nanostructuring – decrease in particle size from a few microns to 50–500 nm, displacement of layers relative to each other and reduction in the number of layers – took place. Spot SAED pattern (inset in Fig. 1a) confirmed the image belonged to the particle of hBN, while that in Fig. 1b indicated the presence of the nhBN nanoparticles with characteristic hexagonal symmetry and NaCl nanoparticles (darker areas on the TEM image) with a cubic lattice.

Fig. 1 TEM images and electronograms of the original hBN (a) and the product of its nanostructuration nhBN (b).

The primary delamination of hBN during the mechanochemical treatment was evidenced by X-ray diffraction data. It followed from Fig. 2 that the intensity of the (002) reflection of nhBN was more than two orders of magnitude less (considering the intensity reduction due to dilution with NaCl) in comparison with the diffractogram of the parent bulk hBN that indicated the particle size reduction in the direction perpendicular to the BN layers.

Fig. 2 X-ray and electron diffraction patterns of the original hBN (a) and nhBN/NaCl mixture (b). Reflexes of NaCl crystals are denoted by asterisks.

We showed previously²³ that during mechanochemical treatment of graphite in the presence of a chemically inert delaminating agent (different inorganic salts were considered), occurrence of defects caused by not only decreasing the number of layers in the graphite particles but also their lateral size due to the breakage of chemical bonds between the carbon atoms in graphene layers and the formation of sufficiently reactive paramagnetic centers its structure was observed. Such centers may interact with water and oxygen molecules and form oxygen-containing functional groups, which contribute to the stability of the resulting graphene dispersions in various solvents.²³ Taking this into account, we hypothesized that similar processes could occur in the case of hBN. To test this hypothesis, we investigated by ESR the samples of hBN and nhBN obtained before and after washing with water within the procedure of the delaminating agent removal. The results showed that the powder of the parent hBN is not paramagnetic. In contrast, after the mechanochemical treatment of the bulk hBN the resulting nanoparticles of nhBN were paramagnetic, the corresponding ESR spectrum being shown in Fig. 3 (curve 1).

Fig. 3 ESR spectra of the mechanochemically treated nhBN/NaCl mixture before (1) and after (2) reaction with water.

The shape of the ESR spectrum of nhBN (curve 1 in Fig. 3) showed that it is a superposition of two multiplet signals due to the hyperfine interaction with ¹¹B nuclei possessing spin 3/2. The first one was a quartet with the hyperfine coupling constant (HCC) equal to 6.8 mT. The multiplicity of the second signal was equal to ten, and the value of HCC was 1.2 mT. According to^28–30 these signals were stipulated by the interaction of the unpaired electron with one or three ¹¹B nuclei, respectively. A sextet with the HCC of 8.7 mT was also present in the ESR spectrum in Fig. 3 (curve 1). It is important that the g-factor of the third and fourth lines of this sextet was accordingly equal to 2.032 ± 0.001 and 1.977 ± 0.003. On the basis of these calculated values, it could be assumed in our opinion that the multiplet is associated with paramagnetic centers Mn²⁺, which were present as an impurity.³¹ When water is added to the nhBN nanoparticles, their paramagnetism disappeared almost completely (Fig. 3, curve 2).

The above results of the ESR studies confirmed our assumption about the formation of the reactive paramagnetic centers in the structure of nhBN during the mechanochemical treatment of hBN, which could react with water and oxygen and form oxygen-containing functional groups. To determine the possibility of modifying nhBN with such groups we studied this nanomaterial (after washing with water) by FTIR spectroscopy, and the obtained data are shown in Fig. 4. It followed from the figure that two characteristic bands about 817 and 1373 cm⁻¹ are present in the spectra of both hBN and nhBN. These bands are related to deformation and stretching vibrations of B–N bonds^19,32–37 and their presence confirmed the retention of the material lattice after the mechanochemical treatment. Importantly, the difference of the FTIR spectrum of nhBN relative to hBN manifested in the appearance of a series of bands in the range of 900–1100 cm⁻¹, which are connected with the deformation vibrations of B–O bonds with different environment.^32–35 Also, stretching vibrations of O–H bonds in B–O–H groups (3412 cm⁻¹)^14,38 and N–H bonds (3200 cm⁻¹)^14,38 were registered (inset in Fig. 4).

Fig. 4 FTIR spectra of the original hBN (1) and nhBN (2).

So, our studies confirmed the presence of the oxygen-containing groups in the nhBN. To our mind, these groups provided the ability to obtain the stable dispersions of gBN after the US disintegration of the nhBN in DMF, EtOH and water. The concentration of the dispersions after centrifugation was respectively 0.55, 0.50 and 0.18 mg mL⁻¹ at the starting loading of the nhBN equal to 1 mg mL⁻¹. We found, by increasing the loading of the nhBN up to 5 mg mL⁻¹, it was possible to obtain the aqueous dispersions with concentration up to 0.9 mg mL⁻¹ under the same conditions of sonication and centrifugation. The yield of gBN could be increased by repeating the procedure of the dispersion/centrifugation of the nhBN. After completing five such iterations the yield of the gBN is about 60% (Fig. 5). The resulting dispersion is stable for 4–6 months. After that period of time the precipitation of the nhBN particles occurred, but they can be redispersed ultrasonically. Preparation of aqueous dispersions of hBN without special chemical modification by direct liquid exfoliation was succeeded in^18,19 earlier. However, in,¹⁸ the value of concentration was only 0.06 mg mL⁻¹, and the 24 hours ultrasonic treatment of hBN at its starting loading of 2 mg mL⁻¹ to achieve a concentration of 0.1 mg mL⁻¹ in¹⁹ was used, whereas the developed method allowed us to obtain at the same loading the dispersion with a concentration of 0.45 mg mL⁻¹ in just 1 hour.

Fig. 5 Dependence of the concentration of gBN aqueous dispersion on the number of successive disintegration/centrifugation iterations at the initial loading of 1 mg mL⁻¹.

TEM images of the particles from the prepared dispersions of gBN in various solvents shown in Fig. 6 made it possible to determine their lateral dimensions which are in the range of 20–150 nm (Fig. 6a–c). Typical SAED pattern of gBN is shown in Fig. 6d. It contains the characteristic reflexes of hBN with the lattice constant of 0.25 nm, which is consistent with the published data,^32,33 and suggests maintaining the crystal structure in the gBN particles after the mechanochemical and ultrasound action.

Fig. 6 TEM images of gBN particles from dispersions in DMF (a), EtOH (b) and water (c); typical electron diffraction pattern for the particles from the prepared dispersions (d).

The prepared gBN was studied by UV-vis and XPS spectroscopy. UV-vis spectrum of the dispersion of gBN in EtOH is shown in Fig. 7. As could be seen from the figure, the maximum of the band due to the interband electron transitions is at 202.5 nm, which is a typical value often found in the literature.^19,36 The linearization of the data obtained by the Tauc equation for direct interband transitions allowed us to determine the value of the optical gap equal to 5.95 eV. This value is substantially greater than the band gap for the monocrystalline hBN (5.77 eV (ref. 34)) that appears to be a consequence of the quantum size effect in the transition from the bulk hBN to its 2D state. At the same time, it is close to the results of other works devoted to the characterization of the 2D-BN samples obtained by the different methods.^25,31,32,36

Fig. 7 UV-vis spectrum of gBN dispersion in EtOH. Inset: spectrum linearization on the assumption of the allowed direct electron transitions.

B1s and N1s peaks of XPS spectrum of the solid gBN particles are shown in Fig. 8. Contributions to the spectra of the different bonds were calculated by numerical simulation. According to the data of,^39–42 components of B1s peak with binding energy (BE) ∼190.0 and ∼191.0 eV correspond to B–N and B–O (hybridized N–B–O) bonds (Fig. 8a) and peak with BE ∼397.9 eV for N1s – to B–N bonds. It should be noted that the shape of XPS spectrum for N1s – possibility of sufficiently accurate modelling by one Gauss–Lorentz curve – indicates that nitrogen atoms in the gBN particles are connected only with boron atoms. Consequently, the oxygen-containing groups on the outer boundary of the gBN nanoparticles, the presence of which was discussed above, are connected with boron atoms, not with nitrogen ones.

Fig. 8 B1s (a) and N1s (b) XPS spectra of gBN.

Raman spectroscopy and AFM were used to study the morphology of the gBN particles. We studied the Raman spectra of the original hBN and the prepared gBN particles using the dispersion in DMF. The obtained data (Fig. 9) indicated the delamination of the hBN with formation of the gBN in dispersion after ultrasonic exfoliation of the mechanochemically nanostructured bulk material. Thus, in the spectrum of the original hBN (Fig. 9a) the E_2g mode associated with vibrations of B and N atoms in the direction from one another within the layer is represented by the band at 1366.3 cm⁻¹ (full-width at half-maximum, FWHM = 10 cm⁻¹). For a thin film of gBN (Fig. 9b), the position of E_2g is shifted to 1368.7 cm⁻¹ (FWHM = 13 cm⁻¹) that is the consequence of the transition of hBN from the bulk in the 2D state according to the results of.^37,38,43,44

Fig. 9 Raman spectra of the original hBN (a) and gBN from dispersion in DMF (b).

AFM data are shown in Fig. 10 and 11. As could be seen from Fig. 10, the lateral size of the particles is from 40 to 150 nm that is consistent with the TEM data presented above. After removing the solvent, the agglomeration of the gBN nanoparticles occurs with the formation of particles with the thickness up to several tens of nanometers. The content of monolayer nanoparticles seems to be small. However, as shown in Fig. 11, with a slow removal of the solvent, in particular DMF, gBN nanoparticles can self-assemble into “nanoroads” with a thickness of ∼0.3 nm and a length up to several tens of microns. The data of ACM could indicate that the proportion of the monolayer nanoparticles of gBN in the resulting dispersion is likely to be the dominant and nano-objects in Fig. 10 and 11 that have a greater thickness apparently also consist of them, in confirmation of the data obtained using Raman spectroscopy.

Fig. 10 AFM images of the gBN particles deposited on the surface of the freshly cleaved mica from dispersions in EtOH (a), DMF (b) and water (c). The thickness of the nanoobjects is denoted by arrows.

Fig. 11 AFM image of the “nanoroads” self-assembled from the gBN nanoparticles.

It should be noted that the formation of “nanoroads” in result of self-assembly of 2D nanoparticles, as far as the author knows, is observed for the first time. More consideration of the phenomenon and the possibilities of its implementation in the case of the other 2D materials will be published by us soon.

The “nanoroads” revealed by AFM microscopy and discussed above were also found out by confocal microscopy (Fig. 12). Due to application of the luminescent mode, the presence of even greater structures is seen in Fig. 12. That became possible owing to luminescent properties of the prepared gBN nanoparticles.

Fig. 12 Confocal microscopy image of the gBN film.

4. Conclusion

Thus, the results of our studies revealed that during mechanochemical treatment of the bulk hBN in the presence of an inert delamination agent the nanostructuring of hBN occurs: its partial delamination, shift of the layers relative to one another, reduction of the particle size. It is shown that the reactive paramagnetic defects are formed in the structure of the nhBN nanoparticles during mechanochemical treatment, and their number is significantly reduced after washing nhBN with water. The possibility of the reaction of such defects with water and oxygen to form oxygen-containing functional groups at the outer edge of the nanoparticles is confirmed by spectral studies, as well as their ability to effectively exfoliate in various organic solvents and in water under the effect of ultrasound. The data of electron diffraction, XPS, UV-vis and Raman spectroscopy indicate the preservation of the hBN structure in the plane of the particles from the resulting dispersions. Predominately monolayer morphology of the particles in the dispersions is confirmed by AFM microscopy and Raman spectroscopy.

Acknowledgements

This work was supported by the Targeted Research & Development Initiatives of the Science and Technology Center in Ukraine and the National Academy of Science of Ukraine and Targeted Comprehensive Fundamental Research Program of the National Academy of Sciences of Ukraine “Fundamental problems of creating new nanomaterials and nanotechnologies”.

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