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. 2019 Mar 11;4(3):5098–5106. doi: 10.1021/acsomega.8b03677

Adverse Effect of PTFE Stir Bars on the Covalent Functionalization of Carbon and Boron Nitride Nanotubes Using Billups–Birch Reduction Conditions

Carlos A de los Reyes †, Ashleigh D Smith McWilliams †, Katharyn Hernández †, Kendahl L Walz-Mitra †, Selin Ergülen †, Matteo Pasquali †,‡,§,∥, Angel A Martí †,§,⊥,*
PMCID: PMC6648908  PMID: 31459687

Abstract

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The functionalization of nanomaterials has long been studied as a way to manipulate and tailor their properties to a desired application. Of the various methods available, the Billups–Birch reduction has become an important and widely used reaction for the functionalization of carbon nanotubes (CNTs) and, more recently, boron nitride nanotubes. However, an easily overlooked source of error when using highly reductive conditions is the utilization of poly(tetrafluoroethylene) (PTFE) stir bars. In this work, we studied the effects of using this kind of stir bar versus using a glass stir bar by measuring the resulting degree of functionalization with 1-bromododecane. Thermogravimetric analysis studies alone could deceive one into thinking that reactions stirred with PTFE stir bars are highly functionalized; however, the utilization of spectroscopic techniques, such as Fourier transform infrared spectroscopy and X-ray photoelectron spectroscopy, tells otherwise. Furthermore, in the case of CNTs, we determined that using Raman spectroscopy alone for analysis is not sufficient to demonstrate successful chemical modification.

Introduction

The study of nanomaterials, materials with at least one dimension smaller than 100 nm, has become a vibrant area of research due to their unusual properties. The unique ratio of surface area to volume of nanomaterials results in novel physical, chemical, electronic, and optical properties.1 In particular, nanotubes are nanomaterials with diameters ≤100 nm and lengths typically in the micrometer range. Specifically, carbon nanotubes (CNTs) and boron nitride nanotubes (BNNTs) have proven promising for a variety of important applications due to their extraordinary properties.

Single-walled CNTs (SWCNTs), which consist of a sheet of sp2-hybridized carbon atoms (graphene) rolled into a tube, are by far the most studied type of nanotube. Their properties include thermal conductivity in the range of 3000–3500 W mK–1 at room temperature2 and Young’s modulus on the order of terapascals.3 Depending on their chirality, CNTs can be metallic or semiconducting,4 with ropes of metallic SWCNTs showing an electrical conductivity of 1 × 104 S cm–1 at room temperature.5 When CNTs are aligned into organized macroscopic materials, such as sheets and fibers, the properties of individual nanotubes can be scaled up and new properties can be attained. CNTs have been used in a variety of applications including as additives in composite materials to increase electrical conductivity, strength, stiffness, and material damping, as coatings to reduce biofouling and corrosion, and as transparent conducting films, electromagnetic shields, and microelectronics.6

BNNTs, structural analogues of CNTs with alternating boron and nitrogen atoms in place of carbon atoms, have been studied less than CNTs but have properties that are just as fascinating. BNNTs have thermal conductivities and mechanical strengths comparable to CNTs,7,8 but unlike CNTs, they are thermally and chemically stable in air at temperatures greater than 800 °C,9 are electrically insulating with a band gap of 5–6 eV,10 and are transparent to visible light.11 These properties complement those of CNTs and allow for a variety of new applications, including as additives in transparent polymer composites and composites in high-temperature oxidizing conditions,12,13 as nanocarriers in medicine,14 and as hydrogen storage media.15

One of the most versatile ways of modulating the properties of nanotubes is by chemical attachment of functional groups. Covalent modification of carbon nanotubes has been explored in many ways; however, one of the most effective methodologies is to use reductive chemistry, which involves the reduction of the CNTs to create reactive conditions that will lead to their chemical modification.16 The production of negatively charged CNTs, or CNT salts, is possible through several methods, such as contact with a molten alkali metal17,18 or using conjugated molecules as electron carriers.19,20 Once the salts are formed, they need to be dispersed in an aprotic solvent for subsequent functionalization upon contact with an electrophile. More direct methods that do not require a dispersion step include electrochemical reduction21 and reaction with organometallic compounds.22−25 Yet another methodology is the Billups–Birch reaction, wherein an alkali metal is dissolved in liquid ammonia to create an electride solution that reduces the CNTs, which then react in situ with an alkyl halide. This one-step method has been used widely to functionalize the walls of CNTs with a variety of molecules.25−31

As stated before, one of the most attractive properties of BNNTs is their chemical inertness toward high-temperature conditions. However, it is this property that frustrates their covalent functionalization. Still, there are a few examples of covalent functionalization of BNNTs, which involve prolonged reflux of the nanomaterials using the functional group as a solvent32 or the reduction of the nanotubes using sodium naphtalide as an electron carrier.33 Recently, we reported the functionalization of BNNTs using the Billups–Birch reduction to afford functionalized BNNTs.34 During our initial experiments, we noticed that BNNTs, which are white by nature, will gain gray color after the reaction. This was accompanied by the emergence of black color on the poly(tetrafluoroethylene) (PTFE) stir bars. Control experiments without the addition of the alkyl halide showed that the material still presents a reasonable weight loss when analyzed by thermogravimetric analysis (TGA) and will also gain dark gray color, even when functionalization was not expected. A potential source of side reactions that could easily be overlooked is the PTFE stir bar. In many publications, the stirring method is not mentioned but could potentially be interfering in functionalization reactions. PTFE has been mentioned to react under the Birch reduction conditions, causing defluorination and darkening of the material.35 This can be easily overlooked in CNT functionalization since the final material is black. In this work, we provide evidence that using PTFE-coated stir bars decreases the degree of functionalization from added grafting groups in carbon and boron nitride nanotubes. Thermogravimetric analysis and spectroscopic techniques are employed to show how the use of glass stir bars enhances the degree of functionalization as compared to that of PTFE-coated stirring. These are important considerations for the functionalization of nanomaterials using strongly reducing conditions.

Results and Discussion

CNTs and BNNTs were functionalized with dodecyl chains using 1-bromododecane through the Billups–Birch reaction employing either glass or PTFE stir bars (see Scheme S1). Bromododecane was used given its long aliphatic chain will allow for a better detection in the different characterization studies. Functionalization was first assessed with thermogravimetric analysis (TGA) under argon. By measuring the weight change as a function of the temperature, we expect to see a loss in the mass due to the desorption of moieties, as well as adsorbates, that we anticipate come from the work-up process. We will analyze the functionalization of CNTs first in the following sections as it is a more studied case.

For pristine CNTs, the thermogram (Figure 1a) shows a flat line when heated under argon since there are no groups to be desorbed from the material. Examining the controls (CNTs that were submitted to the Billups–Birch conditions but without adding bromododecane) with this technique provides us with an estimate of any side functionalization coming from the work-up process. In this case, the control experiments indeed show some weight loss, as can be observed in Figure 1a (Tc-CNTs and gc-CNTs for PTFE- and glass-stirred control reactions, respectively). Given that the control experiments do not involve any added aliphatic groups, upon evaporation of ammonia, unreacted carbon nanotube salts are free to react with any proton source coming from the work-up process, which, in this case are water, ethanol, and hydrochloric acid. Although there are no reports of similar control experiments to support this claim, there are a few examples of reported hydrogenation of carbon nanostructures employing the Billups–Birch conditions using alcohols or water as means of hydrogenation.26,36−38 As such, we suspect that solvent adsorption and hydrogenation could be happening on the nanotube scaffold, causing the loss in weight when heated. As seen in Figure 1a, the functionalized CNTs (Tf-CNTs and gf-CNTs for PTFE- and glass-stirred reactions, respectively) show a distinct behavior, where they display a decrease in weight spanning from 150 °C until 700 °C that is consistent with desorption of alkyl addends. In addition to the initial significant weight loss, the f-CNTs show a slight and constant drop in weight at higher temperatures, possibly from further defunctionalization of more obstructed functional groups.

Figure 1.

Figure 1

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Thermogravimetric analysis and infrared spectra of pristine CNTs, glass-stirred control (gc-CNTs) and functionalized CNTs (gf-CNTs), and PTFE-stirred control (Tc-CNTs) and functionalized CNTs (Tf-CNTs). (a) Thermal profiles and (b) weight loss summary at 900 °C, (c) infrared spectra, and (d) amplification of the aliphatic region.

Interestingly, although the Tc-CNTs lost more weight than gc-CNTs, the reverse happened on the functionalized CNTs. As mentioned before, it has been reported that PTFE reacts with metals such as sodium,39 lithium,40 and even magnesium41 under the Birch conditions. In the same regard, some authors have published that fluorinated nanomaterials undergo defluorination as well.42,43 Chakrabarti and Jacobus studied the reaction of PTFE with lithium in liquid ammonia and reported that the after product in occasions turned dark, which is what happens to the stir bar post reaction (Figure S1). Brecht and co-workers suggested that, unless exposed to oxygen, a foil of PTFE exposed to sodium in liquid ammonia could contain carbon radicals.40 Such findings could indicate that the control experiment using the PTFE stir bar is losing more weight due to possible side reactions with the reactive material exfoliated off the stir bar or due to defluorinated PTFE peeling off from the stir bar into the final product. In the functionalized CNTs, there are two possible causes for the difference in weight loss between the two samples. First, the supply of electrons intended to drive the Billups–Birch reaction for the covalent attachment of aliphatic groups to the CNTs is also reacting with the PTFE stir bar, depleting the source of the reducing agent, which is critical for the reaction to proceed. Previous studies have demonstrated that PTFE under Billups–Birch conditions will result in complex products including fluoride anions, alkyl radicals, and olefins.40 Second, if any alkyl radicals are forming from the PTFE coating, they could also be reacting with 1-bromododecane, which would explain a lower yield in the reaction with CNTs. A plausible competing reaction is that alkyl radicals from detached fragments of the PTFE after defluorination react with CNT. The first derivative of the TGA curves (DTG) of gf-CNT and Tf-CNTs show a broader peak for Tf-CNTs, consistent with a more complex material (Supporting Information, Figure S2). It is important to note that this observation can be easily overlooked, given that the PTFE stir bar exposed to Billups–Birch conditions results in a black coating of the stir bar that could be wrongly associated with CNTs deposition (which would also produce a black coating on the stir bar).

From the thermograms in Figure 1a, we can estimate the number of addends covalently attached to the nanotubes scaffold. With the aid of the control materials, we estimate the extent of side functionalization and adsorbed solvent molecules. Because we are assuming that this weight loss is inherent to the process, the weight loss of the controls was subtracted from that of their respective f-CNTs to obtain the net degree of functionalization due to alkyl moieties. Figure 2b summarizes the weight loss of each CNTs sample measured at 900 °C. After heating, the Tc-CNTs and gc-CNTs lost 18.2% and 12.2%, respectively. Subtracting such values from the functionalized samples, we determined the “corrected” weight loss of Tf-CNTs and gf-CNTs to be 6.8 and 14.4%, respectively (7.6% weight difference). Therefore, we estimate that there is roughly one dodecyl chain per 71 carbon atoms in the gf-CNTs and one chain per ca. 155 atoms in the Tf-CNTs, assuming that all of the weight loss is due to the alkyl chains.

Figure 2.

Figure 2

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Spectroscopic analyses of CNTs. (a) High-resolution X-ray photoelectron spectra (XPS) of the C 1s peak for CNTs, glass-stirred functionalized CNTs (gf-CNTs), and PTFE-stirred functionalized CNTs (Tf-CNTs), as well as the F 1s peak for the latter. (b) Histograms obtained from the Raman mapping showing the D/G ratio at 532 and 785 nm excitation. (c) Two-dimensional plot depicting the Raman defect index (RDI) and Raman homogeneity index (RHI) for the described samples and their control experiments.

As part of the characterization of functionalized nanotubes, we also utilized infrared spectroscopy (Fourier transform infrared, FTIR) to confirm the presence of the C–H stretching vibration (C–Hst) in the region between 2800 and 3000 cm–1. Although the CNTs spectrum is featureless, both gf-CNTs and Tf-CNTs show transitions consistent with aliphatic vibrations (Figure 2c). The amplified aliphatic region in Figure 2d shows that the Tf-CNTs aliphatic vibrations are less intense than those of gf-CNTs, which is consistent with the TGA experiments. The bands between 2250 and 2500 cm–1 are due to CO2 from the environment. Both control experiments show little presence of aliphatic vibrations, consistent with the lack of dodecyl chains on the CNTs.

Complementing the results found by TGA and FTIR, we gained more insight into the chemical bonds found in the CNTs samples by performing X-ray photoelectron spectroscopy (XPS). The high-resolution spectra of the C 1s for the pristine material and f-CNTs are shown in Figure 2a, and the survey scans can be found in Figure S3. We can observe that the C 1s peak for the pristine material was deconvoluted into five components. The most prominent signal at 284.6 eV corresponds to the C=C binding energy and the signal at 285.6 eV to C–C. The next signals, at 286.7 and 288.9 eV, correspond to C–O and C=O binding energies, respectively.44 Finally, at 291.0 eV, we can find the π–π* shake-up feature, attributed to sp2 hybridization.45 The C 1s peak in gf-CNTs was deconvoluted into five components as well, with small shifts in some of the components. Nonetheless, unlike the other two samples, the spectrum of Tf-CNTs shows an additional binding energy at 289.6 eV, which corresponds to a perfluorinated carbon binding energy, so we can safely assume that this peak corresponds to C–F2. We believe that the carbon bound to fluorine found in the C 1s could come from PTFE stir bar residues exfoliated during the reaction. The F 1s peak observed in Tf-CNTs was deconvoluted into two peaks: one at 686.7 eV corresponding to F1–C and one at 689.6 eV corresponding to the F2–C.44 The C 1s binding energies of all samples and their full width at half maximum values can be found in Table S1.

Carbon nanotubes possess sp2 hybridization over their entire structure, except where sp3 hybridized defects occur.46,47 Upon functionalization, we expect that the attachment of alkyl groups causes some rehybridization from sp2 to sp3 (defects in the sp2 network).48−50 Therefore, we calculated the percentage of the total area that each deconvoluted binding energy occupies within the C 1s peak to obtain qualitative understanding of such rehybridization upon functionalization (Table 1). If we calculate the ratio between C=C and C–C, we find that following functionalization both f-CNTs samples have a decrease in the amount of sp2 carbon atoms with a corresponding increase in sp3 carbon atoms. This observation agrees with the nature of rehybridization upon the creation of defects. Furthermore, rehybridization can also be assessed by looking at the change in intensity of the π–π* shake-up satellite. This feature decreases as we go from pristine to functionalized CNTs with a larger decrease seen in Tf-CNTs. This tells us that Tf-CNTs have a greater disruption in their sp2 network than the others, which agrees with the increased intensity of the C–C peak. It is possible that PTFE chains bind to multiple places in the same CNT wall, creating more defects in Tf-CNTs.

Table 1. Percentage of Area of the Binding Energies Deconvoluted from the C 1s Peak of CNTs and f-CNTs.

  CNTs (%) gf-CNTs (%) Tf-CNTs (%)
C=C 61.24 69.98 62.83
C–C 9.05 11.88 16.76
C=C/C–C 6.77 5.89 3.74
C–O 14.58 7.82 3.90
C=O 6.12 4.51 3.56
C–F2 0.00 0.00 10.15
π–π* 9.01 5.81 2.80
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As a final means of f-CNTs characterization, we investigated their chemical modification using Raman spectroscopy by measuring the ratio between the defect-induced D-band (1300–1400 cm–1) and the G-band (∼1600 cm–1).51 Although it is a useful technique, performing single scan points in a sample could lead to large variations; however, scanning Raman spectroscopy encompasses a broader area and provides a better understanding of the characteristics of the material; in this case, an area of 200 × 200 μm2 was scanned for each CNT sample. Hirsch and co-workers first used scanning Raman spectroscopy to interrogate the properties of functionalized CNTs using statistical Raman spectroscopy.52 In their work, the authors scanned an area of the functionalized CNT material and obtained a histogram of the D/G ratio distribution that was fitted with a Gaussian function. From the Gaussian fitting, it was possible to calculate the Raman defect index (RDI) or mean degree of functionalization and the Raman homogeneity index (RHI); for details on this process, refer to the Supporting Information.

Each CNT sample was analyzed with 532 and 785 nm excitation lasers to excite metallic and semiconducting nanotubes, respectively. For visualization purposes, Figure 2b only shows the results for pristine CNTs and f-CNTs and the rest of the samples can be found in Figure S4. Table 2 shows the RDI and RHI values, which can be visually inspected in a plot of I(D/G)532 against I(D/G)785 (Figure 2c). Pristine CNTs have an RDI value of 0.13 and an RHI value of 1187. Any RDI value higher than 0.13 represents a higher degree of defects and any RHI value lower than 1187 would represent a more heterogeneous sample, whereas a higher RHI value would represent a more homogeneous sample. Both of the f-CNTs have a higher RDI, which is consistent with the increase in the C–C binding energy and a decrease in the π–π* satellite in the XPS experiments. It is worth pointing out that the XPS and Raman data demonstrate a higher degree of defects in Tf-CNTs, but this contrasts the barely noticeable C–Hst vibrations in FTIR and relatively low weight loss due to functionalization when heated; thus, in this case, the larger number of defects does not necessarily translate to functionalization with alkyl chains. It is important to remember that not all sp3 defects are generated by covalent functionalization, but they could also be due to the creation of vacancies from the harsh chemical environment.53 In addition, the attachment of carbonaceous aliphatic chains from the stir bar at different sites in the CNTs would lead to features consistent with those visualized by XPS, Raman, and FTIR, since these chains have weaker C–H vibrations but likely attach in multiple places. For the same reason, during PTFE stirring, part of the defluorinated PTFE from the stir bar is likely being attached to the CNTs in addition to the alkyl chains. Comparing the control experiments, we can see that Tc-CNTs are slightly more defective than gc-CNTs, which was expected based on the TGA data.

Table 2. Raman Defect Index (RDI), or Mean Degree of Functionalization, and Raman Homogeneity Index (RHI) of CNT Samples.

  CNTs Tf-CNTs gf-CNTs gc-CNTs Tc-CNTs
RDI 0.13 0.69 0.44 0.25 0.28
RHI 1187 85.91 99.65 80.47 110.4
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Another important feature of the plot presented in Figure 2c is the location of each sample within the plot, that is, in the metallic or semiconducting region. The starting material lies almost on the limit between the two areas, demonstrating that both types of nanotubes are equally defective. All samples, except for Tf-CNTs, lie in the metallic region and that is expected given that those are preferentially modified due to their enhanced reactivity.54−58 Tf-CNTs lie in the limit of the two areas and look as if the CNTs have been randomly modified.

So far, our results indicate that the presence of PTFE in reductive conditions not only hampers functionalization of CNTs but may also mislead one into believing a proper alkylation has occurred. To confirm the effect of PTFE on the functionalization of CNTs using the Billups–Birch reaction, we performed additional control experiments on CNTs following the same procedure as with gf-CNTs and cg-CNTs, except that a porous PTFE membrane was added to the reaction on purpose. Figure S5 shows the characterization of such materials. The FTIR in Figure S5a revealed that only gf-CNTs in the presence of PTFE (gf-CNTs + PTFE) possesses CHst vibrations, which, incidentally, are weaker than any of the f-CNTs described earlier, likely due to competing reactions with PTFE. Moreover, gc-CNTs + PTFE shows a 15% weight loss (Figure S5b), similar to the Tc-CNT weight loss in Figure 1, which is likely due to hydrogenation and PTFE. On the other hand, gf-CNTs + PTFE showed a weight loss of 28%. As expected, this weight loss is larger due to the added functionalization of the bromododecane. Finally, Raman spectroscopy showed that both samples are similarly defective, indicating that PTFE induces defects on CNTs (Figure S5c).

To this point, we have shown that covalent functionalization of CNTs with an electride solution is hampered by using a PTFE stir bar, particularly if haloalkanes are expected to be grafted on the nanotube walls. To further confirm this, we explored the effect of PTFE and glass-covered stir bars on the functionalization of BNNTs using similar techniques. BNNTs intrinsically lack carbon, so any signal from carbon must come from exogenous addends in the reaction mixture. As previously mentioned, because of the black color of carbon nanotubes, the interference of PTFE on the reaction is not detectable with the naked eye. However, because BNNTs are white, the colors of the final products are visually different (see Figure S6), where it is clear that those nanotubes in contact with the PTFE became darker.

The thermograms of all BNNTs samples are shown in Figure 3a. The weight loss of pristine BNNTs is ca. 1.2%, possibly due to adsorbed solvents coming from the purification process (water and ethanol). The rest of the BNNTs samples follow the same trend as the CNTs, wherein the control prepared with the PTFE stir bar loses more weight than the one prepared with a glass stir bar and the opposite trend occurs within the f-BNNTs. The BNNT samples present stepwise losses as well; first, they show a decrease in temperature spanning from 200 °C and then another at ca. 480 °C. From the thermograms, the number of addends covalently attached to the nanotubes scaffold can also be estimated in the same fashion as with CNTs. Figure 3b summarizes the weight loss of each BNNT sample measured at 900 °C and also the net functionalization of f-BNNTs after subtracting the weight loss from the control experiments. The weight subtraction of the controls allows us to estimate the degree of functionalization due to alkyl attachment. As such, we estimated that 15.5% of weight in gf-BNNTs is lost by desorption of alkyl chains, whereas only 7.7% of weight is lost is due to alkyl functionalities in Tf-BNNTs (6.5% weight difference). Using the corrected weight loss values, we estimated that there is one alkyl chain per 36 BN units in gf-BNNTs, whereas there is only one chain per 73 BN units in the case of Tf-BNNTs, assuming that the weight loss is attributed exclusively to aliphatic carbon chains.

Figure 3.

Figure 3

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Thermogravimetric analysis and infrared spectra of pristine BNNTs, glass-stirred control (gc-BNNTs) and functionalized BNNTs (gf-BNNTs), and PTFE-stirred control (Tc-BNNTs) and functionalized BNNTs (Tf-BNNTs). (a) Thermal profiles and (b) weight loss summary at 900 °C, (c) infrared spectra, and (d) amplification of the aliphatic region.

Figure 3c shows the typical infrared spectrum for pristine BNNTs that presents the in-plane B–N stretching at 1350 cm–1 and the out-of-plane B–N bending at 802 cm–1. Upon functionalization, f-BNNTs present a set of peaks corresponding to C–Hst transitions as an additional feature, indicating the presence of alkyl chains. Figure 3d presents an enlargement of the area between 2600 and 3200 cm–1, showing that the C–Hst is significantly more intense in gf-BNNTs in comparison with that in Tf-BNNTs. Also, functionalized BNNTs show a small shift to lower energies in the B–N stretching region that is attributed to lattice deformation in the functionalized materials.33 Even the Tc-BNNTs have some C–Hst vibrations that we believe come from the PTFE stir bar as the gc-BNNTs lack this feature.

Further spectroscopic studies were also performed on BNNTs powders; in this case, high-resolution XPS is shown in Figure 4 and XPS surveys are found in Figure S7; the detailed information for all binding energies are found in Table S2. In the B 1s and N 1s spectra, the pristine material exhibits the common binding energies values of 190.6 and 398.4 eV for B–N and N–B, respectively.59 Additionally, the B 1s and N 1s in the pristine material also show additional small peaks at 192.3 and 399.7 eV, respectively, that have been reported to be due to oxides.60 The B 1s spectra of both f-BNNTs show an increase of the peak attributed to the B–O binding energy, possibly due to further oxidation or functionalization with hydroxyl groups during the work-up process. Moreover, the two f-BNNTs show an additional peak at around 189 eV that correlates to the B–C binding energy;61 it is noticeable that the B–C component in Tf-BNNTs is qualitatively smaller than that in gf-BNNTs. In the N 1s spectrum, there is a slight decrease in the N–O component for gf-BNNTs but we do not discount that nitrogen could also be functionalized, and therefore, this deconvoluted peak could also have some N–C nature. In the case of the C 1s, pristine BNNTs also show a carbon signal but this is likely due to adventitious carbon, as this material did not undergo functionalization. As for the f-BNNTs, they have lost the feature coming from C=O, further confirming that the carbon previously seen in pristine BNNTs is from an adventitious source. Additionally, they both were deconvoluted into a new peak corresponding to the C–B binding energy at ca. 284 eV. Careful inspection of the Tf-BNNTs survey shows that there is a small jump of the baseline at 687 eV, which is the value of F 1s, but because of the low intensity, it could not be analyzed with higher resolution. The low amount of fluorine is not a surprise, given that Birch conditions will cause defluorination of PTFE. It is evident from the TGA and FTIR that the gf-BNNTs are functionalized to a greater extent than the Tf-BNNTs due to the competing role of the PTFE stir bar in the latter case. This is also supported by the XPS, given that the contribution of the B–C component to the B 1s spectra is qualitatively greater in the gf-BNNTs than that in the Tf-BNNTs.

Figure 4.

Figure 4

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High-resolution XPS spectra of boron nitride nanotubes (BNNTs), PTFE-stirred BNNTs (Tf-BNNTs), and glass-stirred BNNTs (gf-BNNTs).

Finally, another feature that can be analyzed from the B 1s peak in the BNNTs samples is the π–π* shake-up satellite, which is also related to sp2-hybridized boron.59 As such, Figure S8 shows that this feature decreases in Tf-BNNTs and even more in gf-BNNTs. Thus, confirming the fact that, upon functionalization, rehybridization occurs.

Conclusions

Here, we present the functionalization of carbon nanotubes and boron nitride nanotubes with alkyl chains using the Billups–Birch reduction. The two reactions were stirred using glass stir bars and PTFE stir bars to demonstrate that the PTFE stir bars hinder the functionalization process, possibly due to competing reactions related to the PTFE coating. TGA analysis of the samples show that the control experiments with PTFE lose more weight than control experiments with glass, indicating that PTFE is contributing to the weight of the material. Conversely, upon functionalization, the glass-stirred samples lose more weight due to a higher degree of functionalization from the alkyl chains. The infrared and XPS spectra also support the fact that the degree of alkyl functionalization is greater in those stirred with a glass stir bar. Additionally, we further confirmed covalent modification with XPS by observing how there is greater sp2 to sp3 rehybridization in both kinds of nanotubes.

It is important to address that the reactions expected from CNT and BNNTs under Billups–Birch conditions are the same. The reason for presenting both cases (CNT and BNNTs) is that for BNNTs, since they intrinsically do not contain carbon, the effect of PTFE is more apparent (see data for Tc-BNNTs vs gc-BNNTs). On the other hand, for CNTs, we used statistical Raman analysis and found that the PTFE-stirred Tf-CNTs are more defective than gf-CNTs. Raman spectroscopy is an easy, fast, and straightforward technique to characterize CNTs, which is the reason why it is widely used to address functionalization. Nonetheless, we see here that a larger defect index is not an absolute proof of greater functionalization; Raman spectroscopy must, therefore, be complemented with other types of techniques.

With this work, we provide evidence that, although it could be easily overlooked, the use of a PTFE stir bar is detrimental to achieving the functionalization of nanostructures under Billups–Birch conditions. To the best of our knowledge, the exclusion of PTFE bars from this kind of experiments has not been addressed previously in the literature.

Materials and Methods

Boron nitride nanotubes (BNNTs) were obtained from BNNT, LLC (P1β type) and purified according to a method published elsewhere.62 HiPco single-walled carbon nanotubes (CNTs) (NanoIntegris HR32-009) were purified following a reported methodology.28 Lithium and 1-bromododecane were purchased from Sigma-Aldrich, and anhydrous ammonia was from Airgas.

Functionalization of Boron Nitride Nanotubes (f-BNNTs)

In a typical experiment, 30 mg of BNNTs (1.2 mmol BN) were placed in a 250 mL oven-dried round-bottom flask. Under vacuum, all glassware was flame-dried. Next, lithium was added (334 mg, 48 mmol) and the system was purged three times with argon. The system was cooled with an acetone/dry-ice bath to condense roughly 125 mL of anhydrous ammonia. The reagents were allowed to exfoliate for an hour with constant replenishment of the cold bath. Later, the cold bath was removed and 2.9 mL of 1-bromododecane (12 mmol) was added dropwise and the ammonia was allowed to evaporate overnight. For the work-up process, the reaction vessel was cooled down with an ice bath and 30 mL of a mixture of water/ethanol (3:1) was added dropwise under argon to quench any unreacted lithium, followed by 10% w/w HCl until an acidic pH was reached. Quenching was followed by extraction with n-hexanes; the organic layer and interphase were collected and washed two more times with water. Later, the product was filtered through a 0.4 μm PTFE membrane and washed thoroughly with more n-hexanes and ethanol, in that order. The material was oven-dried under vacuum at 120 °C overnight before any characterization. Two materials were produced this way, one was stirred using a glass-covered stir bar and another one using a PTFE-coated stir bar to produce gf-BNNTs and Tf-BNNTs, respectively.

Functionalization of Carbon Nanotubes

CNTs were functionalized in a similar way as BNNTs. In this case, 20 mg of CNTs (1.7 mmol) was exfoliated with 473.3 mg of lithium (68 mmol) and later functionalized with 4.1 mL of 1-bromododecane (17 mmol). Likewise, the CNTs that were stirred during the reaction with a glass stir bar produced gf-CNTs and those stirred with a PTFE stir bar produced Tf-CNTs.

Control Samples

Control experiments were prepared using the same conditions as in the functionalization except for the addition of 1-bromododecane to produce gc-CNTs, Tc-CNTs, gc-BNNTs, and Tc-BNNTs.

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 4 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 and a 126 eV pass energy for the surveys. The samples were measured as a powder, and their fitting was performed with MultiPak Spectrum software wherein the backgrounds were subtracted and their peaks shifted in reference to a C 1s value of 284.8 eV. Thermogravimetric analysis (TGA) was performed with a Q-600 Simultaneous TGA/DSC from TA Instruments. The samples were heated under argon to 115 °C and kept at that temperature for 15 min to remove any adsorbates, and were then heated to 1000 °C at a rate of 10 °C min–1. For Raman spectroscopy, the CNTs were analyzed using a Renishaw Raman Microscope RE04 with 532 and 785 nm lasers and mapped in a 200 × 200 μm2 area.

Acknowledgments

We acknowledge the National Science Foundation (CHE 1610175 and 1807737) and AFOSR (FA9550-18-1-0014) for financial support. M.P. acknowledges the financial support of the Welch foundation (C-1668). We thank BNNT, LLC, particularly Thomas Dushatinski for helpful discussions and assistance procuring material for study.

Supporting Information Available

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsomega.8b03677.

  • Conducted reactions, DTG curves for f-CNTs, photography of changes in PTFE stir bars and colors in all BNNTs samples, XPS surveys, detailed information on the high-resolution XPS fitting, Raman spectroscopy of control CNTs, and characterization of control CNTs with additional PTFE (PDF)

The authors declare no competing financial interest.

Supplementary Material

ao8b03677_si_001.pdf (5.1MB, pdf)

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