Quality Assessment of Bulk Boron Nitride Nanotubes for Advancing

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Quality Assessment of Bulk Boron Nitride Nanotubes for Advancing Research, Commercial and Industrial Applications Yadienka Martinez Rubi, Zygmunt J. Jakubek, Maohui Chen, Shan Zou, and Benoit Simard ACS Appl. Nano Mater., Just Accepted Manuscript • DOI: 10.1021/acsanm.9b00057 • Publication Date (Web): 28 Mar 2019 Downloaded from http://pubs.acs.org on April 3, 2019

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ACS Applied Nano Materials

Quality Assessment of Bulk Boron Nitride Nanotubes for Advancing Research, Commercial and Industrial Applications Yadienka Martinez Rubi,†* Zygmunt J. Jakubek,‡ Maohui Chen,‡ Shan Zou,‡ Benoit Simard†

Security and Disruptive Technologies Research Center, National Research Council

†

Canada, Ottawa, Ontario, 100 Sussex Drive, K1A 0R6, Canada ‡Metrology Research Center, National Research Council Canada, Ottawa, Ontario, 100 Sussex Drive, K1A 0R6, Canada

* [email protected]

Boron nitride nanotubes, purity, quality, P3HT, quality index, absorption spectroscopy

ACS Paragon Plus Environment

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ABSTRACT: Evaluating the quality of nanomaterials is essential for both basic research and industrial applications. Here, we describe a methodology for assessment of relative quality of boron nitride nanotube (BNNT) materials by absorption spectroscopy of regionrandom poly(3-hexyl-thiophene) (rra-P3HT) aggregates on BNNTs dispersed in chloroform. The strong selective – interaction between rra-P3HT and nanotubes combined with distinctive visual and spectroscopic changes allows a practical procedure for relative evaluation of BNNT external surface area and surface quality. The method calls first for the identification of the sample-specific polymer loading saturation point, which corresponds to the maximum BNNT surface polymer coverage in the sample. The sample quality (defined as the combination of sample purity and BNNT wall defect density) is determined from the structure and intensity of the absorption spectrum, which depend on the morphology and the total amount of rra-P3HT adsorbed on BNNT samples. This methodology for quality assessment is critical for accelerating the development of new, efficient purification strategies for bulk BNNT materials and advancing the investigation of BNNT properties and industrial applications.


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Boron nitride nanotubes, purity, P3HT, absorption spectroscopy

INTRODUCTION One of the major challenges in characterization of nanomaterials is quality assessment. The lack of reliable methods makes it difficult if not impossible to reproduce and compare results, let alone use the material in applications where the presence of impurities could have catastrophic effects on outcomes. The field of carbon nanotubes (CNTs) has been plagued with this persisting important issue for more than 25 years. A similar issue exists with the emerging boron nitride nanotubes (BNNTs), a structural analogue to CNTs. Owing to the polarity of the B-N bond, the bonding in BNNTs has some ionic character which leads to important property differences with CNTs. Whereas the mechanical and thermal properties are similar, electronic and optical ones are different. In particular, BNNT is a wide band gap insulator, thermally stable in air up to 900 oC, resistant to pure chlorine gas up to at least 1100 oC and totally transparent to visible light. Due to the presence of boron, BNNTs also have the ability to absorb neutron radiation. All these


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characteristics have attracted research interest for different applications of BNNTs in multifunctional structural composites and electronic devices.1,2,3 Despite limited body of research and some inconclusive results, BNNTs are also generally regarded to be biologically compatible and suitable for various biomedical applications.4 Current BNNT bulk production methods generate significant amount of impurities such as boron and BNx derivatives. Unlike for as-produced CNT materials, the majority of impurities in asproduced BNNT materials can be removed by various purification methods. However, the differentiation between BNNTs and BN-containing impurities by common techniques such as Fourier transformed infrared (FTIR), ultraviolet-visible (UV–Vis) absorption and Raman spectroscopies is difficult as they both show similar spectroscopic signatures. Scanning electron microscopy (SEM) and transmission electron microscopy (TEM) are the most common characterization methods used to evaluate purity of BNNT materials but they suffer from a variety of limitations. In particular, SEM and TEM analyses are labor intensive and rarely representative of the bulk material. Furthermore, availability of a new robust quality assessment method is of great importance as complete characterization of


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nanomaterials is usually best achieved by a combination of multiple characterization techniques. Recently, we have shown that regiorandom poly(3-hexyl-thiophene) (rra-P3HT) selfassembles on BNNTs dispersed in chloroform, changing its conformation from coil-like in solution to rod-like of polymer chains stretched along BNNTs.5 The conformational change is spontaneous and is accompanied by distinctive and pronounced colorimetric and spectroscopic changes. While a solution of free rra-P3HT in chloroform appears yellow/orange, fully dispersed rra-P3HT/BNNT in chloroform is magenta/purple colored. Correspondingly, the absorption spectrum changes from a broad unstructured band for the free polymer in solution to a red shifted progression of partially resolved vibronic bands for the suspension of rra-P3HT/BNNT hybrids. These effects were not observed in the presence of hexagonal boron nitride (h-BN), which is the feedstock and one of the main impurities present in BNNT materials produced by the Hydrogen-Assisted Boron nitride nanotube Synthesis (HABS) process.6 The quality of nanotube materials is determined by two parameters: purity and defect density. Purity can be defined as a fraction of nanotubes in a sample, and the defect density can be defined as the


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abundance of structural defects on the nanotube walls.7 Here, we show that rra-P3HT hybridization with BNNTs facilitates a simple spectroscopic and colorimetric method to estimate the relative quality of BNNT materials produced by the HABS process, an essential breakthrough to accelerate the development of applications. We demonstrate that the selective rra-P3HT interaction with BNNTs can be used to determine relative BNNT material quality by measurement and analysis of extinction spectra of fully dispersed rra-P3HT/BNNT suspensions at or near the polymer coverage saturation point, which can be determined by observation of suspension color change or measurement of extinction spectra during dropwise addition of rra-P3HT solution.

EXPERIMENTAL SECTION

Synthesis and purification: BNNTs were synthesized by the Hydrogen-Assisted Boron nitride nanotube Synthesis (HABS) process as described in previous publications.6,

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Small diameter (~5 nm) BNNTs were produced in high yield directly from hexagonal boron nitride (h-BN) powder (99.5%, avg. 70 nm, MK-hBN-N70, M K Impex Corp.). Regiorandom-P3HT (rra-P3HT; MW = 60–90 kD) and regioregular P3HT (rr-P3HT; MW


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= 96 kD, head-to-tail regiospecific conformation > 96%) were obtained from Rieke Metals, Inc. HPLC grade chloroform (OmniSolv) was purchased from EMD Millipore Corporation. The as-produced BNNT material (R-BNNT) was purified through different thermal and solvent treatments to produce three different purified materials (P-BNNTs). P-BNNT-1 material was obtained by purification treatments with chlorine gas at 750 oC as described recently.9

The purified material (gram scale) was dried and homogenized using a

commercial powder grinder. P-BNNT-2 was obtained from P-BNNT-1 after acetone and water washes. P-BNNT-3 material was purified by a process similar to that used to obtain P-BNNT-1 except that the chlorine treatment was done in two steps: first at 750 C and then at 850 C. Subsequently, the sample was washed repeatedly with acetone to remove leftover impurities. The present study provides comparison of these three PBNNTs (P-BNNT-1, P-BNNT-2 and P-BNNT-3).

Thin films: rra-P3HT and rr-P3HT thin films were prepared by drop casting a 0.5 mg/mL solution on a quartz plate and air drying at room temperature.

rra-P3HT methodology for relative quality assessment: The three P-BNNT samples (5.00 ± 0.01 mg each) were prepared and analyzed concurrently. In cases where the P-


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BNNT material was in a compact form, as opposite to a loose powder, the sample was first gently ground using an agate mortar with a small volume of chloroform and then completely transferred to a 100 mL bottle. The methodology is schematically represented in Scheme 1. P-BNNT suspensions in chloroform (0.1 mg/mL) were bath sonicated (ColeParmer 8891) for 15 min to 30 min until no large particles were visible (e.g. BNNT suspension in Scheme 1). A freshly prepared solution of rra-P3HT (0.5 mg/mL) was added gradually in 200 µL aliquots to the P-BNNT suspensions (step1). Bath sonication was applied for 10 min after each addition (step 2), while maintaining the bath temperature in the 22 C to 25 C range, to obtain a stable dispersion of rra-P3HT/P-BNNT hybrids. The addition of rra-P3HT solution was stopped when the color of the suspension appeared to change from purple to brown and the corresponding extinction spectra begun to show increasing absorbance in the region of the absorption band of the free rra-P3HT (near 432 nm). This color change is indicative of an appearance of free rra-P3HT in solution that takes place when rra-P3HT added to the suspension in excess of the saturation point remains free in a coil-like conformation, unattached to BNNTs.5 The polymer coverage saturation point (S) is defined as the rra-P3HT to P-BNNT mass ratio


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corresponding to the onset of the color change. At this point, S provides a first estimate of relative quality: a higher S value indicates a BNNT sample of higher quality. Next, the rra-P3HT/P-BNNT suspension with a minimum amount of rra-P3HT in excess of the saturation point was filtered through a PTFE membrane (1.2 µm pore size, Sartorius Stedim) and rinsed with 15 mL of chloroform to remove free rra-P3HT. A minimum amount or rra-P3HT in excess is recommended to facilitate the effective removal of non-attached (free) polymer by filtration. AFM and TEM verified that no detectable amount of BNNTs passes through the filter, which makes subsequent analyses possible. The wet, purple rra-P3HT/P-BNNT material recovered on the membrane filter was immediately transferred to a 100 mL bottle and re-suspended in 50 mL of chloroform aided by bath sonication for 15 min.


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Scheme 1. Representation of the rra-P3HT methodology for quality assessment of PBNNT materials.

Absorption spectroscopy: The rra-P3HT/BNNT suspensions were characterized at various stages by ultraviolet-visible (UV–Vis) absorption spectroscopy. Spectra were recorded with a Cary 5000 spectrophotometer (Agilent) using 10 mm path length quartz cuvettes.

Analysis of rra-P3HT/BNNT absorption bands: Absorption bands of rra-P3HT/BNNT hybrid in suspension were observed superimposed on a sample dependent scattering background strongly increasing towards the blue and UV. To model the scattering background, various levels of dispersion of BNNT bundles and presence of impurity h-


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BN particles of various size and morphology were considered. We concluded that while more precise description of scattering may be developed in the future when sample morphology is better understood, a simple approximation of the scattering component of the extinction spectra by a function inversely proportional to the to the fourth power of wavelength, 𝐼𝑠𝑐𝑎𝑡 = 𝑎 + 𝑏 𝜆4 ,

𝐸𝑞. (1)

was adequate for reasonably pure samples including those analyzed in this project. The scattering model represented by Eq. 1 was consistent with the Rayleigh scattering approximation which appeared to be approximately valid for suspensions of our samples in the vicinity or above the saturation point. The parameters a and b were adjusted as follows: a was set at the average absorbance value at the long wavelength end of the spectrum, typically in the vicinity of 1800 nm, where the rra-P3HT absorption was expected to be the smallest, and b was set at the maximum value resulting in a nonnegative corrected absorbance in the entire wavelength range in which the extinction spectrum was measured, typically from 250 nm to 1300 nm or 1800 nm. The parameter a mostly accounted for a small detector drift and other instrumental effects usually not


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exceeding 0.005 absorbance unit and for practical reasons could be set in most cases at zero. Pure absorption spectrum of rra-P3HT/BNNT was extracted by subtracting the scattering component from the extinction spectrum. The above-presented simple procedure was expected to be a less accurate description of the scattering component at the short wavelength limit, but sufficiently accurate in the range of rra-P3HT/BNNT absorption bands to the red of 500 nm. While the scattering correction procedure could be avoided entirely by measuring the absorption spectrum with an integrating sphereequipped UV-Vis spectrophotometer or an equivalent dedicated instrument, such accessories and instruments are not always available in general characterization laboratories.

Correlated Atomic Force Microscopy (AFM) and polarized excitation fluorescence microscopy imaging: Samples for correlated AFM and polarized excitation fluorescence microscopy were prepared by spin-coating dilute rra-P3HT/BNNT suspensions (2 g/mL) on piranha-cleaned microscope cover glass (#1, Fisherbrand, Fisher Scientific). Correlated AFM and polarized excitation fluorescence microscopy images were obtained


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using the Nanowizard II BioAFM (JPK Instruments, Berlin, Germany) mounted on an Olympus IX81 inverted optical microscope with an objective-based (UAPO 150 objective, 1.45 NA, oil TIRFM, Olympus) polarized excitation fluorescence setup. The system was enclosed in an in-house fabricated acoustic isolation box and seated on a Micro 60 active vibration isolation table (Halcyonics/Accurion GmbH, Goettingen, Germany). AFM images were recorded in ambient using an intermittent contact mode with silicon AFM tip (HQ:XSC11/AL BS, MicroMasch; typical radius 8 nm, 42 N/m spring constant). First, large size images (100 µm by 100 µm) were recorded to verify the overall morphology and homogeneity of the rra-P3HT/BNNT samples. Next, a series of 10 µm by 10 µm to 40 µm by 40 µm images were acquired at 512 pixel by 512 pixel resolution, with scan rates ranging (0.8–1.0) Hz. Images were flattened using a first-order polynomial fit by the JPKSPM Data Processing software. Polarized excitation fluorescence microscopy images were obtained by illuminating the samples with an LD-pumped allsolid-state 543 nm laser (MGL-III-543, 30 mW, New Industries Optoelectronics Tech. Co., Ltd., Changchun, China). rra-P3HT/BNNT samples were imaged by polarized excitation fluorescence microscopy with a variable direction of polarization in the sample plane


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controlled by a half wave plate (WPH05M-546, Thorlabs) installed in the incident beam path. Fluorescence was collected by the objective (150) and passed through a Semrock FF01-593/40-25 fluorescence filter, resulting in selective detection of the fluorescence in the (570–615) nm range. An optional 1.6 lens was inserted in the detection path for a 240 total magnification of the sample image. The fluorescence images were recorded using a Photometrics Evolve 512 Delta EMCCD camera with the excitation beam polarization rotated in 20 degree steps in the (0–180) degree range with respect to the X axis (left-to-right direction as observed by the analyst in front of the microscope). The fluorescence intensity variation with excitation polarization was analyzed using a custom written ImageJ macro set. The stack of images with varying excitation polarization angle was automatically scanned voxel-by-voxel and fluorescence intensity as a function of the polarization

angle

was

𝐼(𝑥) = 𝐼𝑚𝑖𝑛 +2𝐴cos2 (𝑥 ― 𝛼) ,

fitted

to

Eq.

2

𝐸𝑞. (2)

where 𝐼𝑚𝑖𝑛 is a background corrected minimum voxel intensity, A is the fluorescence intensity peak modulation amplitude, 𝑥 is the excitation light polarization angle, and 𝛼 is the excitation dipole moment angle. Fits yielded the best estimates for the parameters for


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each voxel. In addition, a fluorescence intensity modulation depth parameter, 𝑀 = 𝐴 𝐼𝑎𝑣𝑟 was calculated, where 𝐼𝑎𝑣𝑟 is the background corrected average fluorescence intensity for each voxel.

Scanning electron microscopy (SEM): Samples for SEM were prepared by drop-casting a dispersion of BNNTs in CHCl3 on the SEM pin stub. Images were recorded with a Hitachi High Technologies S-4800v scanning electron microscope.

Dynamic vapour sorption (DVS): Adsorption and desorption isotherms for octane and water vapours were measured at 25 °C on a dynamic vapor sorption (DVS) system (DVS Advantage Instrument from Surface Measurements Systems). This instrument gravimetrically measures uptake and loss of water or organic solvent vapor by a material. Approximately 10 mg of sample was loaded onto the pan balance, before running the software controlled sequence that held the sample at 0 % relative humidity (dried nitrogen) and 100 °C for 10 hours to remove any surface moisture. Then, the relative vapour pressure, p/po, where p and po are the equilibrated and saturation pressures of the adsorptive, respectively, was increased from 0% up to 95% according to the following profile: 2.5% increments below 10% p/po, 5% increments between 10% p/po and 20%


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p/po and 10% increments above 20% p/po. The sample was allowed to equilibrate (dm/dt ≤ 0.0002%/min) at each p/po stage before progressing to the next. The Brunauer-EmmetTeller (BET) equation was applied to the octane adsorption isotherm over the 0.03–0.3 relative p/po pressure range, to determine the specific BET surface area, SBET.

RESULTS AND DISCUSSION P3HT, one of the most extensively studied semiconducting conjugated polymer,10 is an effective dispersant for BNNTs that binds to the nanotubes by - interaction to produce stable suspensions.5 Self-assembly of the polymer chains on the BNNT surface is immediately evident through a color change (Figure 1) and appearance of significantly red shifted, partially resolved absorption bands (Figure S1) with distinguished features at or near 603 nm, 555 nm and 520 nm. These features are characteristic of anisotropic polymer aggregates comprising ordered π-π stacked chain segments.11,12 The three vibronic peaks (An-0, where n refers to the number of vibrational quanta coupled to the excited electronic state) are assigned to A0–0, A1-0, and A2-0 transitions, respectively.8


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Figure 1. Cartoon representation with representative sample images of BNNTs, rra-P3HT and rra-P3HT/BNNT in CHCl3.

A thin film spectrum of rra-P3HT shown in Figure 2a consists of a broad and featureless band (maximum at ~457 nm).

This is because rra-P3HT forms mainly amorphous

(disordered) layers due to the weak tendency for intermolecular packing. The irregular distribution of alkyl side chains in rra-P3HT forces the thiophene units to twist away from planarity, hindering crystallization.13 However, the absorption spectra of rra-P3HT/BNNT hybrids show resolved structure similar to that of regioregular-P3HT (rr-P3HT) films (Figure 2a) and rr-P3HT nanowhiskers.14 Unlike rra-P3HT, rr-P3HT (with a regular head to tail configuration) can readily undergo self-assembly, in both solution and thin film state, resulting in highly ordered two- and three-dimensional polymer architectures.15 Aggregation in rr-P3HT is driven by strong π–π interactions perpendicular to the


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conjugated backbone, as well as by hydrophobic interactions of the side chain, forming a complex microstructure of crystalline and amorphous domains. The absorption spectrum of rra-P3HT/BNNT hybrids indicates that strong - interaction between BNNTs and rraP3HT inhibits polymer chain twisting resulting in polymer planarization along BNNTs with extended conjugation which, in turn, promotes formation of ordered rra-P3HT aggregates (i.e. π-π stacked layers).


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Figure 2. Absorption spectra of rra-P3HT (orange line) and rr-P3HT (purple line) films prepared by drop casting (a); extinction spectra of a rra-P3HT/BNNT suspension in CHCl3 at increasing rra-P3HT to BNNT mass ratio (indicated by the numbers in parentheses) (b); cartoon representation and sample images of rra-P3HT/BNNT (center to right) and rra-P3HT/h-BN (center to left) suspensions at increasing rra-P3HT to BNNT or h-BN mass ratio, respectively (c).

It was found that with continuous addition of rra-P3HT (Figure 2b and 2c), rra-P3HT self-assembles stretched along nanotubes until all available BNNT surface is covered by the polymer5 (e.g. at ~0.08 rra-P3HT to BNNT mass ratio in Figure 2b and 2c). Further addition leaves rra-P3HT free in solution in a coil-like conformation and the color of the suspension appears to change from purple to brown, a result of color mixing (orange colored free rra-P3HT solution admixed to purple colored rra-P3HT/BNNT dispersion). Concurrently, the corresponding absorption spectrum shows increasing absorption at


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~432 nm, which is in the region of the absorption band of the free rra-P3HT in chloroform (Figure 2b, the 0.15 mass ratio curve). The main impurities in BNNTs produced by the HABS process are boron and h-BN derivatives originating either from incompletely vaporized feedstock or produced as byproducts of the HABS process. Other BNH and amorphous BN species are also known to be present.2 The purification process employed here removes essentially all of these impurities except h-BN derivatives. We examined the interaction of rra-P3HT with h-BN (the same used as feedstock) in chloroform and observed that, in contrast to BNNTs, no color change (Figure 2c) nor absorption spectrum shift/change5 was observed upon addition of the orange colored rra-P3HT solution to a suspension of h-BN flakes in chloroform. Moreover, unlike for BNNTs, practically all rra-P3HT could be removed by filtration of the rra-P3HT/h-BN suspension (Figure S2). These experiments indicate that interaction of rra-P3HT with h-BN is much weaker than with BNNTs. It appears that h-BN does not induce planarization of rra-P3HT chains and, consequently, no ordered rraP3HT aggregation is promoted that would lead to distinctive color or spectroscopic changes like those observed for BNNTs. In the same way, Ma et al. have reported only a


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blue shift in the absorption spectrum of exfoliated boron nitride nanosheets (BNNSs) noncovalently functionalized with P3HT.16 To the best of our knowledge, ordered rra-P3HT aggregates with such distinctive vibronic features in the absorption spectrum have only been observed in the presence of BNNTs. Similar experiments evaluating the interaction of rr-P3HT with h-BN (Figure S3) and BNNTs5 showed formation of ordered rr-P3HT aggregates in both materials. The above results suggest that the strong and selective interaction of rra-P3HT with BNNTs and the associated visual and spectroscopic effects could be used to develop a method for relative quality assessment of BNNTs.


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Figure 3. Images of samples of three different P-BNNT materials (P-BNNT-1, P-BNNT-2 and P-BNNT-3) in CHCl3 at increasing rra-P3HT to P-BNNT mass ratio (0.04 to 0.16). Orange rectangle indicates the mass ratio just above the polymer coverage saturation point (S). All samples contain the same initial mass (5.00 ± 0.01 mg) of P-BNNT material at a 0.1 mg/mL concentration.


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The rra-P3HT methodology was applied, in combination with other characterization techniques, to BNNT materials purified by different procedures in order to demonstrate the feasibility of the method. Figure 3 includes images of samples of three P-BNNT materials (P-BNNT-1, P-BNNT-2 and P-BNNT-3) at increasing rra-P3HT to P-BNNT mass ratios prepared as described in the experimental section (rra-P3HT methodology for quality assessment). As mentioned above, when the rra-P3HT to P-BNNT mass ratio exceeds the polymer coverage saturation point (S), the suspension changes its color from purple to brown and the corresponding absorption spectrum begins to show increased absorption in the region of the free rra-P3HT band. Absorption spectra for rra-P3HT/PBNNT-1, rra-P3HT/P-BNNT-2 and rra-P3HT/P-BNNT-3 near and above the saturation point are presented in Figure 4 a-c, respectively. Figures 3 and 4 (a-c) clearly show that different amounts of rra-P3HT were required to “saturate” the coverage for each of the three P-BNNT materials. The absorption spectra in Figure 4 (a-c) also show that the intensity of the partially resolved vibronic bands (with maxima at or near 603 nm, 555 nm and 520 nm) is also different for each material following the trend: P-BNNT-3> P-BNNT2> P-BNNT-1. The rra-P3HT methodology suggests that a higher rra-P3HT to BNNT


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mass ratio at saturation and a higher intensity of the partially resolved bands in the absorption spectrum implies higher BNNT purity (higher BNNT available surface area) since only the BNNT surface significantly induces rra-P3HT planarization and subsequent ordered aggregation. Table 1 summarizes the lower and the upper limits of the saturation point for each P-BNNT samples, with the corresponding specific BET surface areas of PBNNT samples determined from octane adsorption isotherms. In general, there was a correlation between the results obtained by the rra-P3HT results and the surface area trend.


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Figure 4. Extinction spectra of rra-P3HT/BNNT hybrids with rra-P3HT to P-BNNT mass ratios approximately equal to the polymer saturation points (top panel) and SEM images of BNNT samples (bottom panel) for the P-BNNT-1 (a, d), P-BNNT-2 (b, e), and P-BNNT3 (c, f) samples.

Direct visualization of impurities can be obtained by imaging the samples using SEM. Figure 4 shows typical SEM images (Figure 4 d, e, f) obtained for the P-BNNT samples. Additional images are shown in the supporting information in Figures S4 and S5. The trends observed from the rra-P3HT tests (Figures 3 and 4a-c) are consistent with the SEM analysis. P-BNNT-1 with the lowest rra-P3HT to BNNT ratio at the saturation point and the lowest absorption in the region of the spectrum corresponding to aggregated rraP3HT appears to contain significantly higher amount of impurities such as non-tubular structures indicated in Figure 4d. Comparison of absorption spectra of rra-P3HT/P-BNNT2 and rra-P3HT/P-BNNT-3 prompts to a significantly higher purity of P-BNNT-3 since absorbance is higher by a factor of nearly 2 (Figures 4c and 5). However, only slightly


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higher purity for P-BNNT-3 would be inferred from examination of corresponding SEM images. This illustrates one of the limitations of SEM for assessing the quality of bulk samples. Analysis of the features and characteristics of the absorption spectra of rraP3HT/P-BNNT samples sheds light on the observed differences.

Figure 5. Extinction spectra (solid lines), scattering background (dotted lines) calculated using equation 1, and sample images of three rra-P3HT/BNNT hybrid materials in CHCl3 after removal of free rra-P3HT (a). Absorption spectra extracted from the extinction spectra by subtraction of the scattering background (b).


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Figure 5a shows sample images and extinction spectra of the rra-P3HT/P-BNNT suspensions recorded after filtration to remove excess (free) rra-P3HT not adsorbed on BNNT samples, as described in the experimental section and Scheme 1. Although with different scattering backgrounds, the partially resolved bands are observed in all three samples indicating the presence of ordered rra-P3HT aggregates strongly bound to BNNTs. As expected, the absorption band corresponding to the free rra-P3HT is no longer apparent after filtration; the brownish shading disappeared and the color of the suspension returned to purple (sample images in Figure 5a) with the color value of the BNNT suspensions decreasing with increasing sample purity: P-BNNT-3 < P-BNNT-2 < P-BNNT-1. Pure absorption spectra extracted by subtraction of the scattering background (Figure 5a dotted lines) calculated using Eq. 1 are shown in Figure 5b. Subtraction of the scattering background better visualizes additional differences in the absorption spectra of these samples in the 300 nm to 700 nm range. A non-structured high energy region (320 nm to 470 nm) is attributed to nonaggregated or disordered rra-P3HT chains and a lowenergy region (470 nm to 650 nm range) characterized by the partially resolved vibronic


27


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bands corresponds to aggregated rra-P3HT chains.17,18 It is clear from Figure 5b that in P-BNNT-3 the vibronic bands (low energy region) are better resolved than in P-BNNT-1 and P-BNNT-2. This is an indication of a more uniform and narrower conformation distribution of rra-P3HT chains on P-BNNT-3. On the other hand, the higher energy region of the spectrum between 320 nm and 470 nm, corresponding to absorption by disordered rra-P3HT chains, follows the opposite trend with a higher relative integrated intensity contribution observed in the lowest purity material P-BNNT-1 followed by P-BNNT-2. A weaker interaction of rra-P3HT with impurities and also with defective BNNTs present in BNNT samples could lead to adsorbed rra-P3HT chains with significantly disrupted conjugation and no significant contribution to the partially resolved vibronic bands at lower energies. Regioregular P3HT is known to form ordered aggregates composed of π−π stacked chains that can adopt different arrangements and packing. The structural characteristics of aggregated polymer chains have a significant impact on their electronic properties, which can be revealed by optical spectroscopy.19,20,21,22 In this work we observe aggregation of regiorandom P3HT promoted by strong polymer interaction with BNNTs.


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Corresponding absorption spectra are typical of H aggregates. The degree of coupling between aggregated chains is commonly evaluated from the absorption spectra by measuring the intensity ratio of the lowest energy peak (A0-0) and the next replica (A1-0) 8 by using Eq. [3] resulting from the H/J aggregate model developed by Spano and coworkers, where Ep is the energy of the dominant (C=C) intramolecular vibration (0.18 eV) and W is the exciton bandwidth.8

𝐴0 ― 0 𝐴1 ― 0

=

(

1 ― 0.24

𝑊 𝐸𝑝

)

𝑊 1 + 0.073 𝐸 𝑝

2

, Eq [3]

The A0-0/A1-0 peak ratio (Figure 5b) is similar for samples P-BNNT-2 and P-BNNT-3 with approximate values of 0.81 and 0.87, respectively. Poor resolution of P-BNNT-1 spectrum makes reliable determination of the peak ratio more difficult without more rigorous analysis. The similar values of the A0-0/A1-0 peak intensity ratio indicate that rra-P3HT aggregates with a similar degree of order were generated in the samples P-BNNT-2 and P-BNNT-3. On the other hand, as indicated by fractional absorption in the high energy region (320 nm to 470 nm) compared to the total integrated absorption (320 nm to 650


29


Page 30 of 48

nm range) (Figure 5b) a significantly higher fraction of the total rra-P3HT chains adsorbed on sample P-BNNT-2 are in a nonaggegated or disordered conformation as compared to P-BNNT-3. Considering that SEM images appear to show only slightly higher purity for P-BNNT-3 samples, these results suggest that the characteristics of the absorption spectrum of rra-P3HT/P-BNNT samples could be determined not only by the samples purity (fraction of BNNTs in a sample) but also by the sidewall defect density or crystalline quality. The results from the rra-P3HT test (Figures 3, 4 and 5a), supported by SEM and AFM microscopy, indicate the highest purity of sample P-BNNT-3 and additionally the analysis of the morphology of rra-P3HT chains adsorbed on the samples (Figure 5b) suggests that fewer wall defects (higher crystallinity) are present in sample P-BNNT-3, which exhibit the lowest fraction of disordered rra-P3HT. Water adsorption isotherms (Figure S6, right) show decreasing adsorbed amount at 95 % RH on going from P-BNNT-1 to P-BNNT-2 to P-BNNT-3. The water uptake at low relative humidity (i.e. less than 20 % RH) follows the same trend (Table 1), which correlates with increasing sample purity as determined from the rra-P3HT/P-BNNT absorption spectra. The P-BNNT-3 isotherm is characteristic of a hydrophobic material


30

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(e.g. carbon black) with a low water uptake at low relative humidity and a hysteresis loop at higher relative humidity. It can be categorized as Type V in the IUPAC classification.23 Water adsorption behavior can be affected by the presence of polar groups such as functional groups at the surface of the material, as recently shown in a review on carbonaceous materials.24 Therefore, water adsorption on the P-BNNT samples can be explained, in addition to the effect of sample purity, by the presence of varying number of polar functional groups on the BNNT walls and BN impurities that initiate water primary clusters.22 Decreasing density of chemical (functional groups) as well as topological defect sites along the BNNT walls hindering rra-P3HT planarization and aggregation could contribute to a decreasing fraction of disordered rra-P3HT from BNNT-1 to PBNNT-2 and to P-BNNT-3. A less defective (more crystalline) BNNT surface favors stronger interaction and a more homogeneous orientation of the conjugated polymer chains, manifested by more intense and better resolved absorption bands. It is worth noting that other factors such as BNNT diameter and length distribution, level of debundling, and different characteristics of the impurities present (e.g. material density) could also have an effect on the interaction with rra-P3HT and the water sorption


31


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behavior. As an aside, water sorption studies bring attention to the importance of controlling relative humidity in a laboratory when weighing and processing BNNT materials, especially those of low purity and crystalline quality. Polarized excitation fluorescence microscopy has previously been used to determine the conformation and orientation of rra-P3HT on BNNT samples5 and the conformation of P3HT chains in a PMMA matrix.25 However, in the former case identity of BNNTs and impurities was only indirectly inferred from analyses of P3HT fluorescence in polarized excitation fluorescence microscopy images. Representative correlated AFM and polarized excitation fluorescence microscopy images for the rra-P3HT/P-BNNT-3 sample are shown in Figure 6a and 6b, respectively. Additional images of the rra-P3HT/P-BNNT3 sample are shown in Figure S7 and the results of height measurement for samples rraP3HT/P-BNNT-2 and rra-P3HT/P-BNNT-3 in Table S1. An integration of the two microscopic techniques facilitates more direct identification of single and bundled BNNTs, as well as differentiation between rra-P3HT/BNNT hybrids and non-tubular objects (impurities), which allows assessment of the rra-P3HT coverage and morphology on each identified object. As a control measurement, the correlated AFM and polarized excitation


32

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fluorescence microscopy images for the neat P-BNNT-3 sample (without rra-P3HT) are presented in Figure S8.

Figure 6. Correlated AFM (a) and polarized excitation fluorescence microscopy images (b) corresponding to a “polymer saturated” rra-P3HT/P-BNNT-3 sample obtained after filtration to remove unbound rra-P3HT; corresponding parameter mapping images: modulation (c) and fluorescence intensity peak amplitude (d) obtained from voxel-byvoxel fits to Eq. 2.


33


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In the AFM images of the hybrid rra-P3HT/P-BNNT-3 sample (Figure 6 a) more debundled BNNTs (Figure S7), as compared to neat BNNTs (Figure S8), can be clearly identified together with impurities (indicated by red and green ovals). In some cases nontubular structures can be observed attached to an end of a BNNT (blue ovals in Figure 6a), as it has previously been observed by TEM.5 As previously described,5 stacks of polarized excitation fluorescence microscopy images with varying excitation light polarization were automatically scanned voxel-by-voxel, and the florescence intensity as a function of the polarization angle fitted to Eq. 2. A set of five synthetic result images was created with pixel values equal to 𝐼𝑎𝑣𝑟, and best estimates of A, α, M, and r2 of the fit. M and A mapping images are shown in Figure 6 c and d, respectively (Iavr, α, and r2 are shown in Figure S9). Modulation, M, varies from 0 for randomly oriented or coiled P3HT (or stretched P3HT chains crossing at right angles) to 1 for aggregates of oriented P3HT chains. It is evident from the correlated AFM and fluorescence microscopy that most of the AFM identified impurities (non-tubular structures) on AFM images display fluoresce with a modulation depth lower than 0.5 (red ovals in Figure 6 d). Additionally, nonfluorescing impurities (green oval in Figure 6 a and d), were also observed. These results


34

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support the hypothesis that rra-P3HT chains with significantly disrupted conjugation, hence no significant contribution to the vibronic bands in the low energy region of the absorption spectrum that corresponds to rra-P3HT aggregates, are adsorbed on the nontubular impurities. All objects identified by AFM as single or bundled BNNTs showed fluorescence with varying intensity and intensity modulation depth ranging from 0.5 to 1. In the case of BNNTs, a strong interaction with rra-P3HT leads to polymer planarization followed by aggregation of aligned polymer chains and consequently observation of high values of fluorescence modulation depth. A weaker interaction with defective BNNTs present in this sample could be responsible for variation of fluorescence intensity and fluorescence modulation depth values along a length of a nanotube, as observed for some single isolated BNNTs.

Table 1. rra-P3HT coverage saturation (S) range, corresponding BET surface area (SBET), water uptake and rra-P3HT quality index for P-BNNT materials.


35


Material

rra-P3HT to BNNT mass ratio

SBET

Page 36 of 48

Water uptake at

Water

20 % RH

uptake at 20

(m2/g)

(%)

% RH (µmole m-2

rra-P3HT quality index [Qi = I(A1-0)/I420]

g) P-BNNT-1

0.06S0.08

143

5.6

22

1.5

P-BNNT-2

0.08S0.1

199

3.9

8

1.8

P-BNNT-3

0.14S0.16

282

0.7

1

2.6

SBET and water uptake determined experimentally from octane and water adsorption isotherms, respectively (Figure S4)

I(A1-0) and I420: representative absorbance in the low energy region and the high energy region (at 420 nm), respectively (Figure 5b)

Figure 7 illustrates the trends in specific surface area and water uptake at 20% RH as a function of the A1-0 peak rra-P3HT/P-BNNT absorbance for the evaluated samples. The amount of water adsorbed at 20% RH normalized to the unit surface area is also included in Table 1 corroborating a decreasing affinity towards water (i.e. decreasing density of hydrophilic sites) from P-BNNT-1 to P-BNNT-2 to P-BNNT-3 material. As mentioned earlier the rra-P3HT/P-BNNT absorbance increases as the SBET increases. The extrapolation of the linear fit to zero absorbance (Figure 7) indicates that there is 50 %


36

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chance that with no absorption by ordered rra-P3HT aggregates, the BET surface area value is in the range of 35 m2/g to 85 m2/g, thus significantly greater than zero. This range agrees with the SBET values determined for the h-BN precursor (20 m2/g) and impurities (52 m2/g) collected from the acetone washes during the purification of P-BNNT-1.9 Therefore, the intercept or zero absorbance BET surface area is attributed to surface area of impurities. This also supports our stipulation that ordered aggregates, with absorbance at A1-0 peak, are formed on BNNTs only and are not expected to be attached to impurities. The large curvature of the BNNTs compared to the lower curvature of impurities (e.g. h-BN and other BNx derivatives) could be the main reason for the observed selective adsorption of rra-P3HT since only on the nanotubes rra-P3HT chains can orient stretched along the BNNT axis (by π- π interaction) with side alkyl chains wrapping around the tubes.26 In this configuration, unfavorable steric interactions between randomly attached alkyl chains could be minimized and conduce to the formation of rra-P3HT aggregates. It has been shown that when the curvature is reduced π- π interactions increase26,27 hence the selectivity observed in this material cannot be explained by π- π interactions alone but rather by a combination of π- π interactions and Van der Waals forces involving the alkyl chains.


37


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Figure 7. Results from water adsorption isotherms (blue squares) and SBET (red squares) as a function of rra-P3HT/P-BNNT absorbance; red line corresponds to the linear fit and blue line to guide the eye only.

In summary, we observed that: 

rra-P3HT interacts selectively with BNNT to produce a rra-P3HT/BNNT hybrid with unique absorption characteristics that facilitate differentiation from rraP3HT, BNNTs and impurities.


38

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

The amount of rra-P3HT required to titrate the BNNTs, the saturation point S, is directly related to the quality of the BNNT sample with a higher S value indicating higher sample quality.



The absorption spectrum can be divided into a high (320 nm to 470 nm) and a low (470 nm to 650 nm) energy regions corresponding to absorption by disordered rra-P3HT/BNNT brought about by lower crystallinity BNNTs and some complexation with low surface area impurities and ordered rraP3HT/BNNT aggregates formed on BNNT external surface.

Based on the above, to compare quality of different purified BNNT materials (P-BNNT) produced by the HABS method, we introduce a quality index defined as the ratio of absorbance in the low energy region measured at the maximum of the A1-0 band and high energy region measured at 420 nm, [Qi = I(A1-0)/I420]. The absorbance at the low and the high energy regions measures the amount of ordered and amorphous rra-P3HT aggregates, respectively.


39


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CONCLUSIONS We have demonstrated a simple method for assessing relative quality of BNNT samples based on colorimetry and absorption spectroscopy of rra-P3HT self-assembled on BNNTs dispersed in chloroform. The method is based on the observations that rra-P3HT selectively forms ordered aggregates on BNNTs and has no or little affinity with the impurities. Although applied here for BNNT produced by the HABS process, the method should be applicable to BNNT produced using other processes as long as rra-P3HT has no or little affinity with the impurities. Quality assessment of a BNNT material, which is critical for accelerating the development of applications based on BNNT, can effectively be conducted by the methodology proposed in this work. The procedure could also be employed to monitor creation of wall defects by surface treatments, including covalent functionalization. ASSOCIATED CONTENT

Supporting Information. Sample images and absorption spectra of rra-P3HT/h-BN and rr-P3HT/h-BN samples. Additional SEM, AFM and correlated AFM and polarized


40

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excitation fluorescence microscopy images of P-BNNT samples. Water and octane vapors adsorption isotherms.

AUTHOR INFORMATION

Corresponding Author *(Y.M.R.) Email: [email protected]

Author Contributions All authors contributed intellectually and approved the final version of the manuscript. Y.M.R. and Z.J.J. contributed equally.

ACKNOWLEDGMENT This work was supported by the NRC Security Materials Technology Program (Project A1-9469). The authors are grateful to M. Daroszewska for the DVS measurements, to S. Walker and V. Gies for their assistance on BNNT purification, and to K. S. Kim, M. Plunkett and D. Ruth for the synthesis of BNNTs.


41


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Quality Assessment of Bulk Boron Nitride Nanotubes for Advancing

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