Highly-efficient disaggregation and exfoliation of boron nitride nanotubes in aqueous solution using high-frequency focused ultrasonication

Abstract

Boron nitride nanotubes (BNNTs) exhibit excellent thermal, mechanical, and electronic properties, which make them ideal candidates for use as building blocks for functional macroscopic materials. Although efficient dispersion is required for liquid processing of macroscopic materials, reports of highly individualized and high-concentration BNNT dispersions in aqueous solutions are scarce. Here, we investigate BNNT dispersion parameters in order to improve their dispersibility in water with the common ionic surfactant sodium dodecyl sulfate (SDS). Using spectroscopic methods and microscopy, we verified the level of individualization and mass conversion of the BNNTs in dispersion. These dispersions can be concentrated, displaying liquid-crystalline order.

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Introduction

Boron nitride nanotubes (BNNTs) are chemically and thermally stable, mechanically robust, and electrically insulating (∼5.9 eV band gap) one-dimensional nanomaterials.^1–3 BNNTs have a structure reminiscent of that of carbon nanotubes, but instead of a rolled graphene sheet, BNNTs can be thought of as a rolled hexagonal boron-nitride (h-BN) sheet seamlessly connected to form a tube. Due to their unique properties, BNNTs are a promising candidate as building blocks for macroscopic materials, mechanical reinforcements in composites, insulators in electronics, and dielectric thermal conductors.^4–6 Furthermore, they are light, strong, and have good thermal conductivity, which makes them desirable for aerospace applications. However, making use of BNNTs in these applications would require wet processing of the tubes into macroscopic materials, which necessitates their efficient individualization and dispersion in solution. The challenge is that BNNTs are difficult to individualize due to inter-tube attractive van der Waals forces and electrostatic interactions.^1,7 Additionally, although high-concentration dispersions of individualized BNNTs have been reported in superacids (such as chlorosulfonic acid – CSA),⁸ the same is difficult to achieve in aqueous solution. Dispersion in water is safer and easier to implement and would be compatible with a wide variety of processing methods.

Research on BNNTs has gained traction in recent years, building on extensive carbon nanomaterials research. Since carbon nanotubes (CNTs) are structurally analogous to BNNTs, some previously reported methods of dispersion may also be applied to 1D BN nanomaterials. In the current literature, sonication is commonly used for individualizing both CNTs and BNNTs to disrupt the attractive forces between the nanotubes.^6,9–13 Sonication forms small bubbles that cavitate and release high amounts of energy that can disrupt the van der Waals forces between nanotubes.^14,15 However, the implosion of cavitation bubbles can cause scission of nanotubes, especially when using high powered, low frequency instruments such as sonic horns or cup horns.^15–17 This can hinder applications of BNNTs where maintaining their length in dispersion is favorable. Meanwhile, using a sonicator instead with adaptive focused acoustics (AFA) with higher frequencies between 0.5–1 MHz could potentially reduce BNNT scission. This high frequency (compared with the lower frequency of cup horn and probe sonicators of ca. 20 kHz) leads to acoustic waves of only a few millimeters that can be focused into a small zone of the dispersion. This relatively high sonication frequency creates smaller, gentler bubbles, reducing the generation of heat as the bubbles collapse.¹⁸ These bubbles implode with relatively less energy than larger bubbles produced by low-frequency sonication (cup horn and probe sonication), and therefore are typically used as a gentler method for protein¹⁹ and DNA²⁰ extraction. Indeed, studies comparing different sonication frequencies have shown that higher frequencies can reduce defect generation in CNTs.²¹

To aid in BNNT dispersion and prevent reaggregation over time, the use of various dispersants,^6,13,22–29 solvents,^30–35 and covalent functionalization^36–43 in conjunction with sonication have been studied.^3,44 Among these methods, the use of surfactants is of particular interest for the dispersion of BNNTs. Surfactants are amphiphilic molecules containing hydrophobic and hydrophilic regions, are water soluble, and are widely available. In 2002, O'Connell and coworkers demonstrated that sodium dodecyl sulfate (SDS), an anionic surfactant, could be used to individualize nanotube structures, specifically CNTs.²⁴ SDS-dispersed BNNTs have been sorted by length using chromatography.²⁹ However, in this work, researchers reported that dispersions with SDS were not homogeneous after tip sonication. SDS has also been tested to disperse BNNTs in order to assemble composite fibers of BNNTs.⁴⁵ Our group has also investigated the use of anionic, cationic, and nonionic surfactants for dispersing BNNTs and h-BN, indicating that a variety of surfactants were able to individualize BN nanomaterials.^6,46 However, in these previous studies, the maximum reported BNNT yield was only approximately 5% after removing bundles.

More recently, there have been advances towards concentrated dispersions of BNNTs in aqueous solution. In 2023, Lim and coworkers demonstrated that dispersions of BNNTs can be formed without harsh chemicals such as CSA.³⁵ Using polyvinylpyrrolidone and derivatives along with sonication and rotary evaporation, they demonstrated that liquid crystals of BNNTs could be achieved in aqueous solution. Similarly, the surfactant sodium deoxycholate has been used to disperse BNNTs, forming lyotropic liquid crystals in aqueous solution.²⁵ It is important to note that nanotube bundles were not removed in these studies and that the concentrations are based on the amount of BNNT material added.

In this work, we use ultrasonication with adaptive focused acoustics (AFA) to provide high-frequency, gentler sonication to our dispersion. Using this method, we were able to produce high-quality BNNT dispersions in sodium dodecyl sulfate (SDS) with mass conversions (yields) up to 50% percent, corresponding to an over 1000% increase when compared with previous studies using SDS, where BNNT bundles were removed.⁶ Atomic force microscopy (AFM) was used to measure both the length and height of BNNT bundles, indicating well individualized BNNTs after just 24 hours of sonication, which did not appear to exhibit significant shortening with further sonication. Furthermore, spectroscopic characterization shows minimal degree of damage to the walls of BNNTs. Finally, concentrating these dispersions via evaporation drives BNNT self-assembly into liquid crystalline phases, as observed under polarized optical microscopy (POM). This marks significant progress towards dispersion efficiency and the fabrication of macroscopic materials through liquid-phase processing.

Experimental

Materials

All chemicals were used as received from Sigma Aldrich (SDS), or BNNT, LLC (BNNTs). PTFE filters (0.2 μm, 47 mm) were purchased from Advantec. 8 mm glass crimp top vials from ThermoFisher were used as glass inserts for centrifugation. BNNTs were produced by BNNT, LLC (Newport News, VA) by the high-temperature-pressure (HTP) method.⁴⁷ As-synthesized BNNTs were refined to remove residual elemental boron. Further purification etches nanoscale, non-nanotubular BN species, such as nanosheets, nanocages, and amorphous BN, creating highly purified BNNT material,^8,48 which is determined via electron microscopy, including SEM and TEM.⁴⁹

Instrumentation

Sonication was performed using a Covaris S220 Focused-ultrasonicator connected to a VWR 1167P chiller. All absorbance spectra were taken using a Shimadzu 2450 UV-Vis spectrometer. Atomic force microscopy (AFM) images were obtained using a Nanoscope IIIa scanning probe microscope from Digital Instruments, operated in tapping mode. All images were processed using Gwyddion, and length and height profiles were measured using the Profile Tool. To obtain BNNT length, we took the length of the profile drawn as the length of the nanotube. For the height, we subtract the maximum y value and the minimum y value in order to obtain the height. We use this to estimate the bundle size of the BNNTs. Polarized optical microscope (POM) images were taken using a Zeiss Axioscope 5 microscope equipped with a 10x objective, a Zeiss Axiocam 305 camera, and a 10 W white LED for transmitted light illumination. Scanning electron microscope (SEM) images were taken using an FEI Helios NanoLab 660. Fourier transform infrared spectroscopy (FT-IR) spectra were obtained using a Nicolet iS50 FT-IR microscope with an attenuated total reflection (ATR) module. Each spectrum was taken as an average of 64 scans.

Pretreatment of highly purified BNNTs

Approximately 75 mg of a highly purified BNNT puffball (BNNT, LLC) were placed into a glass vial with 10 mL of methanol and stirred vigorously with a PTFE stir bar overnight. This mixture was filtered over a PTFE filter to remove the methanol. The BNNTs were transferred back into the vial, and approximately 10 mL of water were added. This mixture was stirred for at least another 3 hours and then lyophilized. The dried material was used as the starting BNNT material.

Preparation of BNNT dispersions

To prepare the dispersions, boron nitride nanotubes (BNNTs) were combined with a 1 wt% sodium dodecyl sulfate (SDS) solution at a specified weight/volume ratio. For the 1 mg mL⁻¹ starting concentration dispersions, 10 mg BNNTs were added to 10 mL of 1 wt% SDS. Similarly, for 2 mg mL⁻¹ starting concentration dispersions, 20 mg BNNTs were added to 10 mL of 1 wt% SDS. All dispersions were focus ultrasonicated, and the sonication temperature (22 °C) was ensured by a water circulation chiller connected to the instrument. While dispersions were sonicated, 750 μL aliquots were removed at various times. For UV-Vis measurements, aliquots were centrifuged at 8000g for 30 minutes in glass inserts to prevent contamination from the plastic centrifuge tubes in the UV region. The supernatant was taken for analysis. To determine the concentration of BNNTs in the dispersions, a calibration curve was prepared following a previously reported method (Fig. S1).⁶

Preparation of BNNT tip sonicated dispersions

To prepare tip sonicated dispersions for comparison with high-frequency sonication, BNNTs were combined with a 1 wt% sodium dodecyl sulfate (SDS) solution at a specified weight/volume ratio in plastic centrifuge tubes. Using a 1 mg mL⁻¹ starting concentration, 10 mg BNNTs were added to 10 mL of 1 wt% SDS. Dispersions were tip sonicated using a 750 W Cole Parmer ultrasonic processor at 30% power (225 W). After sonication, dispersions were centrifuged at 8000g for 30 minutes and imaged by AFM.

Preparation of AFM samples

Freshly cleaved mica was placed on a hot plate at ∼120 °C and primed with 20 mM MgCl₂. After centrifugation of the dispersions at 8000g for 30 minutes, the supernatant from the BNNT dispersions was transferred to a spray bottle and misted once over the mica surface. Excess surfactant was removed by dipping the mica into water. The mica disc was placed on the hot plate again, and the dispersion was sprayed once more onto the mica. The sample was dipped in water, then soaked in IPA for at least 1 minute to remove any remaining surfactant. Immediately after washing, the sample was dried in an oven at 110 °C for at least 1 hour before imaging.

Preparation of POM samples

To induce the formation of liquid crystals, BNNT dispersions were concentrated using evaporation under N₂ flow. 2 mL of the parent supernatant (obtained from a 2 mg mL⁻¹ dispersion) was placed in a vial and allowed to concentrate under slow N₂ flow. Aliquots of the concentrated dispersion at various times were sampled into 0.1 mm × 1 mm rectangular capillaries and sealed with flame and epoxy. The capillary was bath sonicated for ∼10 seconds to homogenize the dispersion.

Results and discussion

BNNT puffballs were first pre-dispersed by stirring in methanol and water, as described in the Experimental section. Each dispersion was prepared using this material and a 1 wt% solution of SDS. The initial concentration of BNNTs used was either 1 mg mL⁻¹ or 2 mg mL⁻¹, i.e., the SDS/BNNT mass ratio was 10 : 1 or 5 : 1. Controlled focused ultrasonication was used to individualize the nanotubes at a low sonication power of approximately 15 W, and a chiller was used to maintain the water temperature at 22 °C. The focused ultrasonicator used here employs adaptive focused acoustics (AFA), which delivers high frequency sound waves, producing gentler bubbles that reduce heat generation and potentially prevent BNNT scission.

To test the efficacy of focused ultrasonication on BNNT dispersibility, we first determined the optimum centrifugation speed needed to separate undispersed bundles while maintaining a high dispersion concentration. As centrifugation speed increases, more BNNT bundles are separated into the pellet, leaving exfoliated tubes in the supernatant. However, this also decreases the overall concentration of BNNTs in the dispersion. To compare several centrifugation speeds, a sonication time of 24 hours was used for all samples. Then, the average height of BNNTs at different centrifugation speeds (Fig. 1a, and Fig. S2) was assessed using AFM (Fig. 1b and c) to visualize and quantify bundle size. As shown in Fig. 1a, the bundle size decreased slowly from 2000g to 8000g before plateauing. Additionally, the standard deviation of bundle size also narrowed until 8000g and remained relatively constant at higher centrifugation speeds. The average diameter of BNNTs at 8000g in this experiment was 3.4 nm, which is consistent with the diameter of individualized BNNTs. Therefore, 8000g was selected as the optimal centrifugation speed.

Fig. 1 (a) Average bundle size of BNNTs after sonication and varying centrifugation speeds, (b) AFM image, and (c) height histogram taken after 24 hours of sonication and 8000g centrifugation. Scale bar is 2 μm for the AFM image. AFM images at all centrifugation speeds can be found in Fig. S2.

To evaluate the effect of extended focused ultrasonication on BNNT dispersion concentration, BNNTs were added to a 1 wt% solution of SDS at an initial concentration of either 1 mg mL⁻¹ or 2 mg mL⁻¹. Aliquots of 750 μL were removed at several time points and centrifuged at 8000g. After centrifugation, the supernatant was taken for analysis. The concentration of these dispersions was analyzed using UV-Vis spectroscopy, as BNNTs exhibit a characteristic absorbance peak at 204 nm.⁶ In order to extrapolate the concentrations of these dispersions, a previously reported method was used to determine the dispersion yield and construct a calibration curve for BNNTs (see SI).⁴⁶ The extinction coefficient was found to be 530 mL mg⁻¹ cm⁻¹ (Fig. S1). While SDS does not directly interfere with BNNT absorbance measurements, we observed changes in SDS absorbance over sonication time, likely due to decomposition products. As such, the resulting SDS absorbance under the same sonication conditions was obtained separately and subtracted from the dispersion absorbance before calculating the concentration (Fig. S3). Nonetheless, we note that the contribution from SDS byproducts to the absorbance is minimal even at 161 hours (Fig. S4). Fig. 2 shows that the BNNT concentration for both starting concentrations initially increased before reaching a plateau around 80 hours of sonication. The maximum mass conversion for the dispersions at both starting concentrations was between 40–50%, where the mass conversion is the ratio of the concentration after centrifugation and the concentration of BNNTs added at the beginning of the experiment.

Fig. 2 BNNT dispersion concentration with sonication time at 22 °C, with error bars being the standard deviation.

Previous studies have suggested that chemical reactions can be induced on the BNNT surface through sonication over extended periods.^10,50–52 When a cavitation bubble collapses, there is a significant local increase in temperature and pressure.⁵³ These conditions lead to the dissociation of water vapor trapped within the bubble into oxygen reactive species (ROS), including hydroxyl radicals, superoxide, and singlet oxygen.⁵⁴ To assess chemical changes in BNNTs induced by sonication, each aliquot was analyzed by FT-IR to identify changes in the BNNT bonding structure over time. To remove excess surfactant from the BNNTs, the supernatant from each aliquot was filtered over a PTFE membrane and washed with 200 mL of methanol, IPA, and warm water. The BNNTs left on the filter membrane were then re-dispersed in water and lyophilized for analysis. As shown in Fig. 3, the FT-IR spectra show the characteristic BN in-plane vibrational mode at 1355 cm⁻¹ and out-of-plane buckling mode at 790 cm⁻¹.^55,56 The spectra also show a broad signal centered around 3400 cm⁻¹, which may be attributed to –OH groups that form as a result of sonication or may be due to residual surfactant.^50,52 Additionally, the spectra exhibit clear C–H stretching signals near 2800 cm⁻¹, which is likely due to aliphatic carbon from remaining SDS, methanol, or IPA. Interestingly, a peak centered around 1100 cm⁻¹ appears in all sonicated samples and generally increases in intensity with respect to sonication time. This is consistent with defects in the BNNTs that occur after prolonged sonication.¹⁰ Further analysis of these samples using XPS shows a larger percent of oxygen in the sonicated samples. However, the increase in oxygen in the samples was also seen together with an increase in the percentage of carbon, which is likely due to residual surfactant rather than chemical changes on the BNNT surface (Fig. S5 and Table S1). Thus, we conclude that focused high-frequency ultrasonication is a gentle way to disperse BNNTs without significantly disrupting their bonding structure.

Fig. 3 FT-IR spectra of each aliquot after removing excess surfactant. Spectra are normalized to the B–N in-plane vibrational mode at ∼1355 cm⁻¹.

AFM is a powerful tool for assessing the individualization and length of nanotubes (Fig. 4). Previous experiments with carbon nanotubes have shown that ultrasonication can cleave nanotubes, reducing their length in a manner related to the sonication energy (a combination of power and time).¹⁶ It is believed that the axial tension or bending forces caused by the expansion and collapse of cavitation bubbles can lead to BNNT scission.^17,57 To study the effect of sonication on the aspect ratio of BNNTs, the height and length profiles for 100 randomly selected BNNTs imaged by AFM were used to construct histograms at each time point (Fig. 4a–g, S6–S10, Tables S2 and S3). Since BNNTs can range from several nanometers to several microns in length, depending on the synthesis and purification method, it is challenging to compare them with those reported elsewhere. BNNT dispersions were examined from 2 hours to 161 hours of sonication. The samples studied for 2 hours of sonication show tubular structures with lengths of 0.8 ± 0.4 μm, but diameters of 9 nm, indicating that the BNNTs are not well individualized (Fig. S7a). Unfortunately, the length of individual tubes in the bundles cannot be determined, and thus the length reported here is likely inflated due to overlapping BNNTs. The dispersion after 12 hours of sonication (Fig. S7b) shows BNNTs with lengths of 0.7 ± 0.3 μm and diameters of 7 nm. The average length of BNNTs is qualitatively shorter after 12 hours, although the error bars are relatively large. Importantly, among the time points between 24 and 161 hours, the data indicate that BNNT length remains relatively constant, around 0.4 μm (5 nm diameter) for 1 mg mL⁻¹ dispersions (Fig. 4h) and around 0.5 μm (7 nm diameter) for 2 mg mL⁻¹ dispersions (Fig. S8 and S9). Preserving the aspect ratio of BNNTs after sonication is important for various applications, particularly for the manufacture of macroscopic materials. Although there is a slight decrease in BNNT length over sonication time (from 0.7 μm at 12 hours to 0.4–0.5 μm after 24 h), scission appears to be significantly reduced when compared with low-frequency sonication methods such as tip sonication (Fig. S10). A remarkable observation is that when tip sonication is used to disperse BNNTs, delivering the same amount of energy as 161 h of high-frequency sonication, it yields BNNTs with lengths around 0.24 μm. This length is about half the length obtained by high frequency sonication, showing that the latter is less damaging to the BNNTs. Furthermore, Fig. S10b shows the presence of planar structures reminiscent of h-BN sheets together with BNNTs, which are likely due to BNNT unzipping. Given that BNNTs have an average outer diameter of approximately 5 nm,⁴⁷ we are likely observing well-individualized BNNTs after just 24 hours of focused ultrasonication, and further sonication does not affect individualization or nanotube length significantly.

Fig. 4 AFM of BNNTs after (a) 24, (b) 48, (c) 72, (d) 96, (e) 118, (f) 138, and (g) 161 hours of sonication with starting concentration of 1 mg mL⁻¹. Scale bar on all images is 2 μm. (h) Average length and height of BNNTs after sonication, with error bars being standard deviation at each time point.

Wet processing would enable the macroscopic assembly of aligned BNNTs, but it would require first their self-assembly in solution into long-range liquid crystalline (LC) domains. BNNT liquid crystals have been achieved using CSA or high surfactant concentrations in aqueous solution.^8,13,25 However, particularly with BNNTs dispersed with surfactants in aqueous solution, most prior studies utilize BNNTs as received without removing bundles, which can shift the apparent phase boundaries and complicate comparisons across reports. Here, the use of highly exfoliated, individualized BNNTs enables LC phase formation at comparatively low concentrations.

Transitions across the isotropic–nematic regimes were driven by concentrating the supernatant from the 2 mg mL⁻¹ starting concentration dispersion in a vial under a steady N₂ flow. 0.1 mm × 1 mm rectangular capillaries were used to extract an aliquot of the concentrated dispersion at different times followed by ∼10 s of bath sonication of the capillary to homogenize the dispersion. At low concentrations, the dispersion remains isotropic and appears dark under crossed polarizers (Fig. 5a). As evaporation increases both BNNT and surfactant concentrations, the system enters the biphasic regime, where birefringent LC domains nucleate and grow within the isotropic phase (Fig. 5b). Upon further concentration, the fully nematic phase emerges and exhibits textures similar to those reported for BNNT–CSA and BNNT–SDC dispersions (Fig. 5c and Fig. S11).^8,25 The threaded textures and distinct extinction patterns under crossed polarizers are consistent with nematic ordering. Spatial variations in brightness and extinction are consistent with confinement-imposed distortions of the nematic director field in the capillary geometry, leading to spatially varying orientation and optical retardance across adjacent domains. As summarized in the phase schematic in Fig. 5d, we observed the onset of the biphasic region around C_i = 3.42 mg mL⁻¹ and the transition to a fully nematic phase at C_n = 7.14 mg mL⁻¹. While these trends are consistent with prior BNNT–SDC observations with critical concentrations below predicted by Onsager theory,^25,58 the critical concentrations reported here are even lower, which may be attributed to improved exfoliation and higher BNNT purity.

Fig. 5 Phase behavior of BNNT–SDS dispersions concentrated under N₂ flow and examined by cross-polarized optical microscopy (POM); (a) isotropic phase (1.0 mg mL⁻¹), (b) biphasic region (4.3 mg mL⁻¹), and (c) fully nematic phase (16.5 mg mL⁻¹). Scale bars represent 100 μm. (d) Schematic representation of the phase diagram as a function of BNNT concentration, identifying approximate isotropic (○), biphasic (×), and fully nematic (●) regimes.

Because these phase boundaries were approached via solvent evaporation in a confined capillary, the observed textures can depend on how quickly and uniformly concentration gradients homogenize during solvent removal. The static nature of this evaporation process can introduce kinetic limitations. Concentration gradients, particularly near the solvent–air interface, may locally crowd the dispersion, and produce a jammed, kinetically trapped state. Therefore, although small regions of birefringence are present before capillary sonication (Fig. S12), larger domains with birefringent textures emerged only after brief bath sonication of the capillary (Fig. S11). This suggests that mechanical perturbation is needed to merge the nematic regions, and bath sonication appears to be sufficient to re-mobilize the nanotubes and allow relaxation toward an equilibrium nematic morphology.

Conclusion

In summary, focused-ultrasonication (low energy, high frequency) was used to disperse BNNTs and achieve concentrations over 1 mg mL⁻¹ (a 40–50% mass conversion). In particular, dispersion experiments using 2 mg mL⁻¹ initial concentration show that good exfoliation and dispersion can be achieved around 48 hours, providing a convenient, scalable and mild strategy to disperse BNNT in aqueous solution. FT-IR and XPS studies indicated that BNNTs undergo minimal chemical changes after sonication. AFM images show that BNNTs are well-individualized and do not exhibit significant shortening over long periods of low-power sonication after 24 h, in contrast with reports using low-frequency sonication. Concentrating the solutions by evaporation under a N₂ flow reveals the formation of liquid crystalline phases at relatively low concentrations, and enables estimation of the phase diagram boundaries.

Conflicts of interest

The authors declare the following competing financial interest(s): Lyndsey R. Scammell is the Product Development Manager at BNNT, LLC, which has commercialized the synthesis of boron nitride nanotubes. The remaining authors declare no competing financial interest.

Data availability

The data supporting this article have been included as part of the supplementary information (SI). Supplementary information is available. See DOI: https://doi.org/10.1039/d6nr00896h.

Specific data files (such as instrument files or data spreadsheets) related to this study are available upon request from the corresponding author.

Acknowledgements

We thank BNNT, LLC for their assistance in procuring materials for this study. We acknowledge the NSF-BSF (2404270) for financial support.

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