Chemical Decoration of Boron Nitride Nanotubes Using the Billups

Article Cite This: ACS Appl. Nano Mater. XXXX, XXX, XXX−XXX

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Chemical Decoration of Boron Nitride Nanotubes Using the BillupsBirch Reaction: Toward Enhanced Thermostable Reinforced Polymer and Ceramic Nanocomposites Carlos A. de los Reyes,† Kendahl L. Walz Mitra,† Ashleigh D. Smith,† Sadegh Yazdi,# Axel Loredo,† Frank J. Frankovsky,† Emilie Ringe,†,¶,# Matteo Pasquali,†,¶,§,# and Angel A. Martí*,†,‡,¶ †

Department of Chemistry, §Chemical and Biomolecular Engineering, #Materials Science and NanoEngineering, and ‡Bioengineering and ¶Smalley-Curl Institute for Nanoscale Science and Technology, Rice University, Houston, Texas 77005, United States S Supporting Information *

ABSTRACT: The combination of properties of boron nitride nanotubes (BNNTs) makes them desirable building blocks for the development of functional macroscopic materials with unprecedented electronic and mechanical features. However, these properties have not been fully exploited because their chemical inertness hampers their processing. One solution is to covalently functionalize the BNNTs to assist in their individualization, dispersion, and processing. Here, we show that dodecyl chains can be covalently attached to BNNTs through the Billups-Birch reaction using lithium and 1-bromododecane as reagents. By combining thermogravimetric and spectroscopic analyses, we were able to verify the presence of the alkyl chains that chemically graft to the outermost wall of the nanotubes, as well as unveil the sp2 to sp3 rehybridization. The hydrophobic addends change the dispersibility and individualization of BNNTs in various organic solvents, which we envision will allow the manufacturing of sophisticated materials such as polymer and ceramic nanocomposites with enhanced strength and thermal stability. Furthermore, because of the inherent thermal stability of BNNTs, the alkyl moieties can be easily removed at high temperatures in air without oxidizing the nanotubes. This chemical functionalization provides a straightforward way to tune the properties of BNNTs, which until now has proven to be a formidable undertaking. KEYWORDS: covalent functionalization, boron nitride nanotubes, Billups-Birch reaction, Birch reduction, alkylation, nanotubes dispersion

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chirality.5 Nonetheless, carbon nanotubes are not the right material for many applications: they lack an electrically insulating type, have a low thermal stability in air,6 and are not transparent in the visible part of the electromagnetic spectrum. A material with these properties, such as boron nitride nanotubes (BNNTs), would complement carbon nanotubes and is necessary for the next generation of functional materials created from these building blocks. Boron nitride nanomaterials are structurally analogous to carbon nanomaterials due to the fact that the boron−nitrogen

INTRODUCTION

In 1996, the Nobel Prize in Chemistry was awarded for the discovery of buckministerfullerene (buckyball), a cluster of 60 carbon atoms forming a close icosahedral structure.1 The discovery of fullerenes led to the emergence and rapid growth of a new area of research: nanoscience. Fullerenes are considered the simplest of the fullerites, and soon after their discovery, new structures known as carbon nanotubes also emerged.2 Carbon nanotubes (CNTs), which are a few nanometers wide and hundreds of nanometers long, are the ultimate building block for making advanced materials that are lighter than aluminum but with a tensile strength stronger than that of steel,3 have a high thermal conductivity,4 and can be either conducting or semiconducting depending on their © XXXX American Chemical Society

Received: April 17, 2018 Accepted: May 4, 2018

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DOI: 10.1021/acsanm.8b00633 ACS Appl. Nano Mater. XXXX, XXX, XXX−XXX

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ACS Applied Nano Materials bond is isolobal to the carbon−carbon bond. BNNTs have an architecture equivalent to CNTs but with alternating boron and nitrogen atoms occupying the positions of carbon (Figure 1).

In this work, we propose to use the Billups-Birch conditions to functionalize multiwall BNNTs, a method that has proven successful for the covalent functionalization of carbon materials with different functional groups.24−26 Unlike the procedure reported by Shin and co-workers,23 this method does not require the previous reduction of nanotubes since the exfoliation and reduction occur in situ for subsequent alkyl functionalization. Furthermore, the product of our functionalization reaction shows remarkable infrared transitions that are proof of chemical modification. We demonstrate that the alkylated BNNTs show a reduction in sp2 hybridization due to the formation of a covalent bond with the alkyl chains and that these alkyl chains are located on the outer wall of the nanotubes. These functionalized BNNTs show increased dispersibility in long-chained aliphatic solvents, while their dispersibility in water is severely diminished.

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MATERIALS AND METHODS

Materials. Boron nitride nanotubes were purchased from BNNT LLC (batches #310172 and #627172); lithium and 1-bromododecane were purchased from Sigma-Aldrich. Boron nitride nanotubes were purified with a method reported by Chen et al.27 Preparation of Functionalized Boron Nitride Nanotubes (fBNNTs). Oven-dried BNNTs (30 mg, 1.2 mmol) were functionalized following a modification of the Billups-Birch reaction.25 Briefly, the material was added to an oven-dried and flame-dried three-neck round-bottom flask (250 mL) with lithium (334.6 mg, 48 mmol), as illustrated in Scheme 1. The system was purged three times with

Figure 1. Representative structure of carbon nanotubes and boron nitride nanotubes with alternating boron and nitrogen atoms.

BNNTs have attracted a lot of interest during recent years due to their combination of unique properties. Besides possessing a high mechanical strength comparable to that of carbon nanotubes,7 BNNTs also possess a uniform wide band gap,8 have a robust thermal conductivity,9 and have a high chemical and thermal stability.10 Pristine BNNTs have been used in applications ranging from reinforcements for composite materials11,12 to a variety of biomedical applications.13 To further exploit the outstanding properties of BNNTs and use them in novel applications, macroscopic BNNT materials must be fabricated. However, the incredible chemical and thermal stability of BNNTs coupled with their low dispersibility critically hinders the production of advanced materials. Bulk functionalization, which allows for the tuning of these properties, is one route to resolving this problem. Through the addition of functional groups, bulk functionalization can assist in the individualization and dispersion of nanotubes in a variety of solvents. Functionalization of BNNTs is of particular importance in the manufacture of composite polymer and ceramic materials since the functional groups can be tailored to enhance the compatibility of the materials. This opens new routes for obtaining novel materials with improved chemical, thermal, and optoelectronic properties. While several publications have addressed the chemical grafting of boron nitride nanosheets (BNNSs),14−21 strategies for the covalent functionalization of BNNTs are scarce. For example, the attachment of a stearoyl group to BNNTs was first achieved by refluxing CVD-grown multiwall BNNTs in stearoyl chloride at a high temperature for 5 days.22 However, this functionalization method requires amino groups on the edges or on the outermost wall of the BNNTs to work; the amino groups would function as anchorage for stearoyl chloride to make an amide bond. Unlike their precursor, the newly modified BNNTs were dispersible in various solvents such as tetrahydrofuran (THF), N,N-dimethylacetamide, and toluene. Alternatively, using DFT simulations, Shin et al. showed that covalent functionalization is in theory favorable on reduced tubes and performed preliminary experiments on this front.23

Scheme 1. Preparation of Functionalized Boron Nitride Nanotubes (f-BNNTs), Control BNNTs, and Impregnated BNNTs

argon, and then anhydrous ammonia was allowed to condense until it reached an approximate volume of 125 mL. Then, the mixture was stirred with a glass stir bar for an hour to allow for exfoliation of the nanotubes followed by dropwise addition of 1-bromododecane (2.9 mL, 12.1 mmol). The mixture was allowed to react overnight; during this time, the ammonia slowly evaporated. Following the evaporation of the ammonia, the flask was cooled down in an ice bath and 40 mL of cold water−ethanol (3:1) was injected under an inert atmosphere. The mixture was then acidified with 10% w/w HCl to dissolve the lithium salts and extracted with n-hexanes. The f-BNNTs will slightly disperse in hexanes but will mostly migrate to the hexane−water interface. The interface and organic layers were washed several times with water, filtered through a 0.4 μm PTFE membrane, and washed with more n-hexanes and ethanol, in that order. Two control materials were also prepared (see Scheme 1). The sample described simply as “control” consists of BNNTs that underwent the procedure detailed above except for the addition of 1-bromododecane. The “impregnated” control was prepared by stirring BNNTs with the same equivalents of 1-bromododecane for 24 h and then washing them in B


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ACS Applied Nano Materials the same way that a normal functionalization reaction would require. All materials were dried under vacuum overnight in a furnace at 150 °C before any characterization. Preparation of AFM, XPS, and STEM Samples. Dispersions of 0.25 mg/mL of the materials in THF were prepared and sonicated for 30 min and then centrifuged at 16 000g. The supernatants were collected, and the sonication and centrifugation steps were repeated twice more. The final supernatant was drop-casted on glass for AFM; for XPS, drop-casting was repeated on glass until a thin white film was visible on the surface (approximately 60 drops). For STEM, additional dispersions of the samples were prepared in the same way in isopropanol. The final supernatant was drop-casted on lacey carbon grids with no Formvar (300 mesh) and dried for 3 days under vacuum at 185 °C to remove any source of carbon contamination. Dispersibility Measurements. The dispersibility of all samples was tested in water, isopropanol (IPA), tetrahydrofuran (THF), methylethylketone (MEK), dodecane, and cetane (hexadecane) by preparing dispersions (0.25 mg/mL) and measuring the absorbance of the supernatant after three rounds of sonication and centrifugation at 16 000g. Scattering due to dispersed nanotubes in different solvents was measured at 285 nm. Due to the long cutoff wavelength of MEK, dodecane, and cetane, the absorbance was measured at 345, 312, and 300 nm, respectively; from the spectrum of BNNTs in water, we obtained a ratio between 285 nm and each of these wavelengths that was applied to the three spectra to obtain a projected value at 285 nm. Instrumentation. Fourier transform infrared (FTIR) spectra are the average of 64 scans recorded using a Nicolet 6700 with an ATR accessory and a resolution of 1 cm−1. X-ray photoelectron spectroscopy (XPS) spectra were obtained with a PHI Quantera SXM Scanning X-ray microprobe system using a pass energy of 26 eV for high-resolution spectra. The samples were drop-casted from a THF dispersion onto glass, and the peak energies were calibrated with the Si 2p peak from SiO2 centered at 1072.5 eV. Peak fittings were performed using the MultiPak Spectrum software. Thermogravimetric analysis (TGA) was performed with a Q-600 Simultaneous TGA/DSC from TA Instruments. The samples were heated under air until 115 °C and kept at that temperature for 20 min to remove any adsorbates and then heated up to 1000 °C at a rate of 10 °C/min. AFM measurements were performed with a Bruker Multimode 8 AFM system in tapping mode using ScanAsyst Air silicon cantilevers. Transmission electron microscopy was performed with a JEOL JEM 1230 TEM operated at 80 kV. Scanning transmission electron microscopy electron energy loss spectroscopy (STEM-EELS) and high angle annular dark field STEM (HAADF-STEM) were performed on a FEI Titan Themis3, equipped with a Gatan Quantum ERS electron spectrometer, operated at 300 kV. UV−vis spectra were obtained with a UV−vis Spectrometer Shimadzu UV-2450; in the case of diffuse reflectance measurements, an integrating sphere was attached to the UV−vis spectrometer and the samples were diluted with BaSO4 (1% w/w).

electrons to concentrate on these sites. As a result, it can be assumed that functionalization will primarily, but not necessarily exclusively, occur on the boron atoms. Bulk Characterization. As explained in the Materials and Methods section, functionalization was performed by condensing liquid ammonia over a mixture of lithium and BNNTs followed by the addition of 1-bromododecane. In order to demonstrate an effective functionalization of the BNNT framework, we first used spectroscopic techniques such as FTIR. The spectra of four materials, pristine BNNTs, the control, the impregnated BNNTs, and the f-BNNTs, as well as 1-bromododecane, are presented in Figure 2a. All BN samples

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RESULTS AND DISCUSSION Functionalization of Boron Nitride Nanotubes. During the Billups-Birch reaction, the reduction of the nanotubes by lithium plays two key roles: exfoliation and enhancement of reactivity. In our particular case, the reduction of BNNTs may lead to exfoliation due to electrostatic repulsion between the nanotubes, making them more available for reaction (similarly to CNTs).28−32 Additionally, the highly reductive environment increases the reactivity between the alkyl bromides and the otherwise inert nanotubes. In short, an electron transfer reaction takes place upon the addition of the alkyl halide, forming a radical anion that later dissociates and creates an alkyl radical, as proposed by Billups, which will in turn attack the nanotube framework to produce alkyl-functionalized nanotubes. This will have the concomitant effect of sp2 character loss.25,33,34 Given that the boron sites are lower in electron density than the nitrogen sites, we expect the surplus of

Figure 2. Bulk characterization. (a) FTIR of BNNTs, control BNNTs, functionalized BNNTs (f-BNNTs), and BNNTs mixed with 1bromododecane and then washed (impregnated BNNT), and 1bromododecane. (b) Tauc plot of pristine BNNTs, control BNNTs, and f-BNNTs obtained by diffuse reflectance. (c) Weight loss as a function of temperature of BNNTs, control BNNTs, f-BNNTs, and impregnated BNNTs.

have two characteristic peaks around 1340 and 790 cm−1 in common, which correspond to the in-plane B−N stretching and the out-of-plane B−N bending, respectively.35 Unlike the other three, f-BNNT is the only sample with absorption peaks in the 2780−3020 cm−1 region, which match the C−H stretching vibrations in 1-bromododecane. Another noticeable feature from the infrared spectra of f-BNNT is the appearance of peaks at 1090 and 1150 cm−1. The first is consistent with the C


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ACS Applied Nano Materials

Figure 3. Spectroscopic characterization of the chemical bond. High-resolution XPS spectra of B 1s and N 1s for (a) BNNTs and (b) control BNNTs. (c) High-resolution XPS spectra of B 1s, N 1s, and C 1s for f-BNNTs. Insets in all B 1s spectra show the π-bond shake up satellite related to sp2 hybridization. The table shows the values for each binding energy and their full width half-maximum values.

values reported for the sp3 environment in cubic boron nitride,36,37 which is indicative of a change in hybridization due to the attachment of the alkyl moieties to the boron atom. The latter vibration has been reported to correspond to a B−C bond.21,23 Another observed vibrational mode is the transition around 960 cm−1, which seems to be present in all the materials but has a greater intensity in the f-BNNT. Moreover, it is worth mentioning that the B−N characteristic vibration at 1340 cm−1 is shifted 5 and 9 cm−1 in the control and f-BNNT, respectively, which has been related to lattice deformation.23 Finally, it is noteworthy that the impregnated BNNTs lack any of the additional features aforementioned, implying that the presence of the C−H vibrations are not due to adsorption of the 1bromododecane to the BNNT surface. Further spectral characterization of these materials was made by measuring the diffuse reflectance (Figure S1) in the solid state, and the results can be observed in a Tauc Plot in Figure 2b. One of the features in the plot is the shift of the main peak from 201.3 nm in pristine BNNTs to a lower energy, 202.9 nm, in f-BNNTs (6.16 to 6.11 eV), a 1.6 nm difference, which has been previously attributed to the creation of new electronic states within the bandgap.22 Additionally, there is a new shoulder in both the f-BNNT and control BNNT at 214.5 nm (5.78 eV). Finally, by extrapolating the linear region of the main peaks to the energy axis, we can obtain the band gap value from the intercept. This method was used to determine that, following functionalization, f-BNNTs experience a decrease of

0.11 eV in their bandgap, a trend also observed by Sainsbury et al. for BNNSs functionalized with dibromocarbene.21 Thermogravimetric analysis was used to provide an estimate of the bulk functionalization of the f-BNNTs. The thermogram in Figure 2c shows a 1.1% weight loss in the pristine material possibly due to solvent molecules adsorbed during the purification process. The control and the impregnated samples lose only 1.7%, which could account for solvent molecules trapped in the innermost tube. We do not discard that the weight loss in the control could also come from small amounts of functionalization from the workup, i.e., protons or hydroxyl groups. It is known that, once a carbon nanomaterial salt is formed, addition of water or an alcohol will lead to hydrogenation,38−40 a situation that could also be happening in this case. The 6.6% weight loss in the f-BNNTs with an onset at 274 °C indicates the combustion of the carbon chains. Accounting for the 1.7% loss observed in the control, we calculated that the weight loss due to the carbon moieties in the f-BNNTs was 4.9%. This percentage represents 1 chain per 131 BN units (or 1 chain per 262 atoms) in BNNTs. It is important to emphasize that we modified multiwalled BNNTs, which impacts the apparent degree of functionalization as we are only modifying the outermost wall. Indeed, in functionalization of SWCNTs31 and MWCNTs41 with decanoyl acid moieties using potassium naphtalide as reducing agent, Pénicaud and coworkers found through gravimetry that, as the number of walls increased, the degree of functionalization decreased from 1 functional group per 80 carbon atoms to 1 per 125 carbon D


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Figure 4. Structural characterization and atomic distribution. Low resolution TEM of (a) pristine BNNTs, (b) control BNNTs, and (c) f-BNNTs. HAADF-STEM and STEM-EELS line profile of (d) a pristine BNNT and (e) a f-BNNT.

189.1 eV which, given the lower electronegativity of the carbon atom compared to nitrogen, corresponds to B−C bonding.45 More importantly, the decrease in the intensity of the π-bond shake up satellite in this sample indicates sp3 rehybridization, which is consistent with the FTIR results. The N 1s binding energy was deconvoluted into two peaks corresponding to the N−B bonding featured in the pristine material and a small contribution from N−C bonding. The C 1s peak was deconvoluted into three peaks that are consistent with B−C, C−C, and C−N bonding at 283.6, 284.8, and 285.9 eV, respectively. Nonetheless, carbon signals were present in all samples including BNNTs and control BNNTs likely due to adventitious carbon. Therefore, no significant interpretation should be attributed to the carbon data of such samples. Microscopy Studies. The structural changes in the BNNT walls with functionalization were observed by TEM (Figure 4a−c). The pristine and control BNNTs have similarly intact walls; however, the sidewalls of the control are more rugged. In contrast, the walls in f-BNNTs have defects; the roughness of the walls could be either holes or due to partial unwrapping of the outer BN tube layer. Most of the nanotubes observed in fBNNTs present these features, meaning that the presence of defects is characteristic of functionalization, which is supported by the early oxidation observed in the TGA. In CNTs, it has been observed that the presence of defects in the pristine material enhances reactivity.46,47 Similarly, it is easy to imagine that, once there is any defect in BNNTs, functionalization starts to occur, yielding more edges where further chemistry can happen.

atoms. Our degree of functionalization of 1 chain per 131 BN units indicates at least 1 chain per nm in a three-walled BNNT. We predict that single-walled BNNTs would yield a better degree of functionalization. Lastly, looking at the oxidation stability of the materials gives us an insight into the functionalization. BN materials convert to boron oxides when heated in air at temperatures over 800 °C. It is noteworthy that the onset of oxidation decreased by approximately 54 °C in the f-BNNTs compared to pristine BNNTs, a behavior also observed in MWCNTs due to the introduction of defects into the framework.42 A reasonable explanation is that increasing functionalization leads to an increase in defect sites, which oxidize more readily than a more pristine material. Characterization of the Boron−Carbon Bond. To further study the nature of the bonds in f-BNNTs, XPS was performed. BNNT samples were dispersed in THF (as explained in the Materials and Methods section), and films were drop-casted on glass. The high-resolution spectra of pristine BNNTs (Figure 3) shows typical values for B 1s and N 1s at 190.8 and 398.3 eV, respectively, due to B−N bonding.43,44 For further analysis of the other samples, the energy and fwhm values of B 1s and N 1s were fixed in order to find other components during the deconvolution of the signals. Similar to pristine BNNTs, the control presents peaks for B 1s and N 1s binding energies without any extra features. Moreover, these two materials present what is known as the π-bond shake up satellite, due to the sp2 hybridization.43 In the f-BNNTs spectra, additional traits can be observed. For example, the B 1s spectrum clearly shows a new shoulder at E


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Figure 5. Individualization of nanotubes. AFM and height profiles of (a) BNNTs, (b) control, and (c) f-BNNTs drop-casted on glass in THF after three rounds of sonication and centrifugation.

Using STEM-EELS, we investigated the distribution of functionalization across the nanotubes; the K-edges for boron (188 eV), nitrogen (401 eV), and carbon (281 eV) present in the samples can be seen in the EELS spectra in Figure S2. It is noteworthy that, in both the pristine BNNT (Figure 4d) and the f-BNNT (Figure 4e), the ratio of boron to nitrogen is roughly one to one, as expected. However, the distribution of carbon differs. The relative composition plot shows that, in the pristine BNNT, the carbon content is concentrated in the center of the tube; this carbon most likely comes from solvent molecules trapped in the innermost tube. In contrast, the carbon in the f-BNNT is concentrated on the edges of the nanotube. This behavior is expected for carbon grafted onto the surface of the nanotube as the edges of the tube provide more probing points due to the curvature of the nanotube. Attachment of the carbon chains to the nanotubes affects not only their thermal stability but also their dispersibility in solvents. As explained in the Materials and Methods, after three rounds of sonication and centrifugation, the obtained supernatants of the dispersed nanotubes in THF were drop-casted on glass and imaged using atomic force microscopy (AFM). Although all three samples were treated with the three rounds of sonication and centrifugation, the topographical images in Figure 5 reveal that only f-BNNTs consist of individual nanotubes. Furthermore, only f-BNNTs do not show impurities such as hexagonal boron nitride sheets, in contrast with the other two. The height profiles give us an idea of the degree of nanotube entanglement, i.e. higher values, such as those present in the pristine BNNTs, are due to bundling of nanotubes and sheets. For each sample, 100 nanotubes were individually identified and analyzed by measuring the height at their centers. This analysis showed that the majority of the nanotubes in fBNNTs have diameters between 6 and 8 nm, while the pristine and control samples have a wider distibution of diameters (Figure S3). Such an outcome implies that nanotubes with smaller diameters tend to get functionalized more than those with larger diameters. This result is similar to that of CNTs,48 where the increased surface strain of the nanotubes with smaller diameters increases their reactivity. Alternatively, this effect could be just due to a better exfoliation for f-BNNTs than for pristine BNNTs and control. Dispersion Performance. The results from the dispersibility experiments are depicted in Figure 6, where for each solvent, the absorbance was normalized to that of the pristine

Figure 6. Change in the dispersibility of BNNTs, control BNNTs, and f-BNNT in different solvents normalized to pristine BNNTs.

BNNTs for direct comparison against the dispersibility of fBNNTs. The non-normalized plot, which gives an idea of the dispersibility efficiency of different solvents for the different samples of BNNTs, is presented in Figure S4. Pristine BNNTs and control BNNTs readily disperse in water, yielding a very cloudy dispersion. In contrast, f-BNNTs aggregate on the meniscus of the vial immediately after sonication due to increased hydrophobicity upon addition of the aliphatic chains; this visual observation is in agreement with the absorbance results. IPA represents a solvent in which the dispersibility of the three is similar, taking into consideration that, although the hydroxyl group is present, the carbon chains might help the fBNNTs to disperse. Interestingly, of all the solvents investigated, THF showed enhanced control BNNTs dispersibility. In the case of MEK, dodecane, and cetane, the f-BNNTs disperse better than pristine BNNTs. Unlike dodecylatedCNTs, which readily disperse in solvents such as chloroform and hexanes,25 it is difficult to draw a trend for the f-BNNTs due to their different nature. For the former, we have a pristine material that is hydrophobic, and for the latter, we have a material that is partially charged due to the different electronegativities between B and N. So, attachment of the chains to the f-BNNTs does not necessarily yield a material that is completely nonpolar. However, one trend that could be seen among the tested solvents is that the length of the carbon chains seems to play a role, given that the dispersibility of the fBNNTs was found to be related to the length of the aliphatic carbon chains of the solvents. We believe that this could be due F


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Figure 7. Characterization of BNNTs after removal of the carbon chains. (a) TGA of f-BNNTs that were heated up to 600 °C and then heated from 100 °C up to 1000 °C (restored BNNTs); the TGA also shows the TGA of pristine BNNTs as a comparison. (b) FTIR of pristine BNNTs, fBNNTs, and restored BNNTs. (c) AFM of restored BNNTs.

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CONCLUSIONS In summary, we demonstrated that the Billups-Birch reduction conditions can be effectively used to functionalize BNNTs with aliphatic carbon chains, adding a very straightforward and fast methodology for the chemical functionalization of such inert materials. TGA analyses showed that heating the f-BNNTs resulted in both a weight loss due to oxidation of carbon chains and a decrease in the oxidation temperature onset, indicating defects in the network. Spectroscopic techniques such as FTIR and XPS have revealed that this procedure covalently attached carbon chains to the BN network, and diffuse reflectance analysis showed a band gap reduction upon functionalization. With TEM, we perceived changes in the BNNT morphology after functionalization, and STEM-EELS indicated that carbon is grafted onto the surface of the f-BNNTs. An appealing property of the f-BNNTs is that the presence of the alkyl chains leads to increased exfoliation and dispersion in different solvents; specifically, f-BNNTs showed a higher dispersibility in long-chained solvents. Moreover, if assembled into a more complex material, the carbon functionalities could be easily removed from the f-BNNTs upon heating in air, yielding unmodified BNNTs. This study provides a way of effectively functionalizing BNNTs with aliphatic groups showing important modulation of the properties of the BNNTs and potentially of the macroscopic materials manufactured with these building blocks. The grafted group on the surface of the fBNNTs is expected to improve the compatibility of BNNTs with polymers and ceramics allowing the manufacture of composite materials with great thermal stability, chemical resistance, and high durability.

to better intermolecular interactions between the aliphatic chains on the tubes and the chains from the solvents. Defunctionalization. In addition to developing a methodology for functionalizing BNNTs, we are interested in addressing whether this functionalization could be removed. Since BNNTs show remarkable temperature stability, we investigated whether we can selectively burn the hydrophobic chains to regenerate the pristine material. To investigate this, we heated the f-BNNTs in the TGA instrument at 600 °C for 30 min in order to burn off the functional groups and then we allowed the material to cool down to 100 °C. Afterward, we ramped the temperature up to 1000 °C. During the first temperature run (up to 600 °C), the sample lost 7.3% in weight; however, during the second temperature ramp, no weight loss was observed, mirroring the behavior of the pristine material (Figure 7a). The conversion of f-BNNTs to BNNTs was also investigated with FTIR. After heating the f-BNNTs at 600 °C in a furnace (to burn the alkyl chains), the recovered material was analyzed by FTIR (Figure 7b). Interestingly, we found that the material lost the vibrations due to C−H stretching and the bands attributed to the sp3 hybridization of boron markedly decreased. Thus, not only were the aliphatic groups removed, but also part of the sp2 hybridization was recovered. The restored BNNTs were dispersed in THF and analyzed by AFM, which revealed tubular BNNT structures, although some degree of delamination can be occasionally seen (Figure 7c). This result demonstrates that, due to the high stability of the BNNT core, alkyl chains can be selectively burned off to recover nonfunctionalized BNNT materials. These discoveries have important implications for the processing of BNNTs into functional materials. G


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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsanm.8b00633. Diffuse reflectance and UV−vis measurements, as well as EELS spectra and histograms of the diameters of individual nanotubes obtained by AFM (PDF)

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected] (A.A.M.). ORCID

Emilie Ringe: 0000-0003-3743-9204 Matteo Pasquali: 0000-0001-5951-395X Angel A. Martí: 0000-0003-0837-9855 Author Contributions

A.A.M. conceived the idea. C.A.d.l.R. designed the experiments mentored by A.A.M and M.P., functionalized the BNNTs, performed the FTIR, TGA, diffuse reflectance, AFM, XPS, TEM, and water contact angle experiments, and wrote the manuscript. K.L.W.M. executed the functionalization numerous times and helped on the characterization and writing of the manuscript. A.D.S. purified the nanotubes and designed the idea about the AFM of the restored BNNTs. S.Y. and E.R. performed the microscopy on STEM and helped analyze the data. F.J.F. and A.L. helped with the dispersibility experiments and UV−vis measurements, as well as prepared AFM and XPS samples. Funding

We acknowledge the National Science Foundation (CHE 1610175) and AFOSR (FA9550-18-1-0014) for financial support. M.P. acknowledges the financial support of the Welch foundation (C-1668). Notes

The authors declare no competing financial interest.

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Chemical Decoration of Boron Nitride Nanotubes Using the Billups

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