Mechanistic Consideration of pH Effect on the Enrichment of

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Mechanistic Consideration of pH Effect on the Enrichment of Semiconducting SWCNTs by Conjugated Polymer Extraction Jianfu Ding,*,† Zhao Li,† Jacques Lefebvre,† Xiaomei Du,‡ and Patrick R. L. Malenfant*,† †

Security and Disruptive Technologies Portfolio, and ‡Energy, Mining and Environment Portfolio, National Research Council Canada, M-12 and M-50, 1200 Montreal Road, Ottawa, Ontario K1A 0R6, Canada S Supporting Information *

ABSTRACT: Enrichment schemes providing high-purity semiconducting single-walled carbon nanotubes (sc-SWCNTs) will enable their implementation into high end and printed electronics. Conjugated polymer extraction (CPE) has been shown to be a very effective and scalable method to isolate sc-SWCNTs with purities >99.9%. However, this method is often plagued with variability, and the mechanism is not thoroughly understood. Herein, we probe the origins of selectivity in polyfluorene-assisted enrichment using poly(9,9-di-n-dodecylfluorene) (PFDD) and find that the affinity of the wrapping polymer to bind metallic (m-) and semiconducting SWCNTs is similar and may not contribute to the selectivity, but rather that oxygen-driven p-doping of the nanotubes and its dependence on the surface acidity may play a vital role for the selective dispersion of sc-SWCNT. The latter hypothesis is tested by titration experiments in which sodium hydroxide (NaOH) is used to neutralize the SWCNT surface, thus mitigating oxygen p-doping of SWCNTs that arises via the oxygen/water (O2/H2O) redox couple. The selectivity of the enrichment is completely lost when 1 equiv of NaOH per 70 SWCNT carbons is used. Oxygen doping is believed to be the driving force that triggers the aggregation of highly polarizable mtubes. These novel observations stress that care must be taken during purification schemes prior to enrichment using CPE, which is strongly influenced by acid/base treatments, likely due to the pH dependence of p-doping via the O2/H2O redox process.

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INTRODUCTION Conjugated polymer extraction (CPE) is an effective technique for scalable sc-SWCNT enrichment, providing purities greater than 99.9%.1−3 However, CPE suffers from batch to batch fluctuations in yield and purity, due likely to impurities introduced in different purification schemes used prior to enrichment and by uncontrolled environmental factors such as ambient humidity.4 Gui et al. have recently reported that selectivity for CPE in nonpolar, hydrophobic solvents is significantly affected by the presence of trace water and the redox state of the SWCNTs.5 We have also shown that the variability can be mitigated by adding a conditioning step, which is effectively an extraction process but at a low conjugated polymer (CP) concentration to remove undesired species on the SWCNT surface prior to enrichment.4 Hence, variability observed in sc-SWCNT enrichment can be attributed to the presence of impurities as well as oxygen and water from ambient air, which can alter doping level and the charged state of the nanotubes.4,5 This is not unexpected given that SWCNTs are highly environmentally sensitive materials with their electronic and spectroscopic properties strongly affected by ambient moisture and oxygen.6−14 It is estimated that oxygen/water (O2/H2O) redox doping generates 1 hole per 77 carbons under ambient conditions,10 and the doping level is sensitive to pH on the nanotube surface.8,14−18 In solution, the doping of nanotubes will affect their interactions with the © XXXX American Chemical Society

dispersing medium, the wrapping polymer, substrates and nanotubes themselves, especially with highly polarizable mtubes.18−23 The polarizability difference between m- and sctubes has been ascribed as the driving force that enables selectivity in nanotube enrichment.1,3,4 Furthermore, selective interactions and/or enrichment have been observed on chemically modified substrate surfaces,24−33 with dispersants,34 or in chromatography,17,35 and aqueous two-phase extraction.5,36,37 However, the role of doping and pH on CPE and the implications on the polarizability model has yet to be examined. Herein, we find that the affinity of PFDD in binding metallic and semiconducting SWCNTs is similar, indicating that selectivity of the enrichment by PFDD extraction originates from a different mechanism. Rather, a loss of selectivity observed upon neutralizing with NaOH suggests that oxygendriven p-doping of the nanotubes via the O2/H2O redox process modulates nanotube−nanotube interactions via the high polarizability of m-tubes. NaOH titration neutralizes the SWCNT surface, impedes oxygen p-doping, and precludes the selective interaction of doped nanotubes (both metallic and semiconducting) with the highly polarizable m-tubes, leading to a loss of enrichment selectivity. Therefore, we demonstrate that Received: June 13, 2016 Revised: September 3, 2016

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Figure 1. Absorption spectra of the acid-treated SWCNTs sample dispersed in PFDD/toluene solution (raw) and the supernatants for three successive PFDD extractions from the acid-treated SWCNTs sample (A1, A2, and A3) (a), and from the nontreated SWCNTs sample (N1, N2, N3) (b); and Raman scattering curves of the RBM region under 785 nm laser excitation (c), and D & G bands under 633 nm laser excitation (d) for raw, A1, and N1. The supernatants were obtained by dispersing 25 mg (dry weight) of SWCNTs sample from the previous centrifugation in 25 mL of PFDD (1 mg/mL) solution using 30 min horn sonication (mini-tip, cycle 60% and output 30%) at ∼30 °C, followed by a centrifugation at 12 500 rpm using SS-34 rotor (RCF 18 690) for 30 min.

CPE is strongly influenced by acid/base treatments, due to the strong pH dependence of p-doping via the O2/H2O redox process.

value is from the enrichment using an acid-treated plasma SWCNT sample, which was obtained by stirring a raw plasma SWCNT sample in a 4 N HCl solution for 40 min and then washing with pure water until the filtrate was neutral. However, when the same enrichment process was applied without acid treatment (nontreated sample), the sc-purity of the extracted product was relatively lower with the ϕ value dropping to 0.38, corresponding approximately to a 1% sc-purity decrease from 99.9%. Figure 1 compares the absorption spectra of the supernatants from the first three successive extractions for both acid-treated and nontreated raw materials (successive extractions are performed on the precipitate from the prior step). The spectra show similar features for both products with an absent metallic absorption band (M11) typically centered around 700 nm. However, spectra for the nontreated sample displayed a slightly higher background absorption in this region, indicating a lower purity. The Raman spectra for all three extractions have the same features, and thus only the spectra from the first extractions are compared to that of the raw material in Figure 1c and d. The enriched sample from the nontreated material displayed a weak broad band around 160 cm−1 in the RBM region under a 785 nm laser excitation and a G−m peak at around 1545 cm−1 with a broad Breit−Wigner−Fano line shape in the D & G band under 633 nm laser excitation (shadow area). These two bands are attributed to m-tubes according to the Kataura plot, based on the fact that tubes derived from the

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RESULTS AND DISCUSSION Previously,4 we reported that the enrichment of plasma torch derived SWCNTs using PFDD extraction provides an excellent semiconducting (sc-) purity with our purity metric, ϕ, reaching over 0.41. It corresponds to 99.9% sc-purity based on Raman mapping.4,38 ϕ is calculated from absorbance versus wavenumber and is defined as the ratio of the integrated area without the trapezoid background over the whole integrated area under the S22 and M11 bands.3 This value usually varies from 0.05 to 0.43 with the increase of sc-purity from plasma SWCNTs. This value can sensitively reflect the sc-purity change at a high sc-purity level such as 99% sc-content. Usually at this level, the M11 band in the absorption spectrum is absent, and thus the traditional purity estimation based on the relative peak intensities of M11 and S22 bands does not apply. However, any small sc-purity change at this level will still lead to a background change and thus can be sensitively reflected by ϕ value. However, because many other factors such as the presence of bundles, defects, doping of nanotubes, impurities such as amorphous carbon, and catalysts will also affect the background, using the ϕ value for sc-purity assessment is valid only when the same types of materials are compared. This high ϕ B

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Figure 2. Effect of base-treatment on enrichment: Absorption spectra of the supernatants for three successive PFDD extractions from the basetreated SWCNTs sample (B1, B2, and B3) (a). Absorption spectra of the acid-treated SWCNT sample dispersed in PFDD/toluene solution (raw), and the supernatants of the third extraction (Ex3) from the NaOH-titrated samples at NOH/NC ratios of of 0.000, 0.001, 0.0024, 0.0048, 0.0096, and 0.0144 (b). Raman scattering curves of these samples under 785 nm laser irradiation for the RBM region (c) and 633 nm laser irradiation for the D & G region (d). The titration is achieved by adding 0.02 mL of NaOH solution at a concentration of 0.08, 0.2, 0.4, 0.8, and 1.2 M into a suspension of the acid-treated SWCNTs (20 mg) with PFDD (10 mg) in 25 mL of toluene followed by 1 h bath sonication to facilitate a thorough interaction of NaOH with the nanotubes.

plasma torch synthesis have diameters centered at 1.3 nm.4,38 Therefore, the low ϕ values of the enriched products from the nontreated SWCNTs sample indicate the existence of a small amount of m-tubes. To explore the cause of the lower purity for the enrichment of the nontreated sample, the supernatants of the first extraction from both nontreated and acid-treated materials (N1 and A1) were examined by transmission electron microscopy (TEM). Their TEM images (Figure S1) show that star-like structures (spherical masses with tubes emanating from the surface) that existed in N1 were completely removed in A1, indicating the acid-treatment removed the spherical particles, and thus a thorough separation of m- and sc-tubes can be expected for A1 (thermogravimetric (TGA) data shown in Figure S2 further corroborate this observation). However, the yield data listed in the inset of Figure 1 show that PFDD extraction from the acid-treated sample is not as efficient, with only about 50−70% of the yield from the nontreated sample. Improved selectivity and decrease in yield of this magnitude suggest that the acid treatment may be doing more than just removing the star-like structures linking both metallic and semiconducting SWCNTs, whereby other effects such as surface charge generation via p-doping may be occurring. This effect was verified by neutralizing the acid-treated sample by stirring in 0.5 M NaOH solution for 30 min followed by washing with distilled water until the filtrate was neutral.

Interestingly, enrichment of the base-treated sample displayed a significant decrease in sc-purity. UV spectra in Figure 2a of the supernatants for three successive extractions from this sample display an apparent M11 band around 700 nm with a ϕ value reduced to 0.31−0.33, a value much lower than for the acidtreated sample (0.406−0.416).4,38 This result clearly shows that the surface acidity of the nanotubes has a significant effect on the enrichment.39,40 A similar phenomenon has also been observed in density gradient ultracentrifugation separation, whereby oxygen doping in an acidic medium is described.41,42 It is interesting to note that both acid- and base-treated samples were washed with plenty of water until neutral, so that a nearly neutral surface is expected on the nanotubes in both cases. However, the significant difference in the enrichment purity from these two samples indicates that their surfaces are distinguished and may possess slightly different acidity and surface charge. It is well-known that the SWCNT surface typically has defects such as COOH moieties. This functional group can be converted to acidic (COOH) or basic forms (COO−) after acid or base treatment, respectively. While the pH level of water on the surface is largely determined by the dissolved CO2 from the ambient, approximately 6.0,15 the pH on the acid-treated samples can be slightly lower than this value due to the existence of COOH, thus further enhancing pdoping. On the other hand, for the base-treated samples, the dissociation of COONa will not only raise the pH value on the C

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Figure 3. Variation of ϕ values of the supernatants of the first, second, and third extractions (Ex1, Ex2, Ex3) (a), and their average ϕ value and overall yield as a function of NOH/NC ratio (b). The yields were calculated from UV spectra on the basis of the weight of starting materials using a method reported in ref 3.

gradually became clear again after approximately 30 min, indicating NaOH powder had dissolved and reacted with the SWCNT surface under sonication. A complete reaction at the nanotube surface may take a longer time; therefore, the data from the second and third extraction should more closely reflect the chemical equivalent of the titration because the 30 min horn sonication used for these extractions will further facilitate this reaction. While the ϕ value was reduced from the highest value of 0.43 for the acid-treated sample to a value close to the starting material (∼0.10) as NaOH usage increased from 0.000 to 0.024 (N OH /N C ), the yield of material in the supernatant concomitantly increased from ∼1.4% to 24% (Figure 2b). This result clearly indicates that the acid-treated nanotubes have a strong capability to aggregate and form bundles that can be drawn into the sediment by centrifugation at 18 690g. These bundles preferentially combine m-tubes and thus yield a high selectivity for the enrichment. However, the enrichment on the base-titrated sample at a high NaOH usage (i.e., NOH/NC = 0.024) resulted in complete loss of selectivity and an extremely high yield (24%) in the supernatant, indicating that under these conditions, both sc- and m-tubes can be well-dispersed in the medium, and were stable enough to stay in the supernatant under the 18 690g centrifugation. It should be noted that both single tube dispersion and small bundle dispersion are possible in this sample. 18 690g is only a moderate centrifugal force, which might not be strong enough to draw small nanotube bundles into the sediment. To verify the possibility of m-tubes existing in the supernatant as small bundles will be critical in our succeeding discussion, hence we have applied two additional successive ultracentrifugations at 48 000 and 105 000g to the supernatant from the base-titrated sample (NOH/NC = 0.024). The absorption spectra of the supernatants and redispersed sediments under 18 690, 48 000, and 105 000g are compared in Figure S4. As the centrifugal force increased from 18 690 to 48 000g, the yield decreased from an extremely high value of 24% to a normal value of 3.0%, which is typically observed for PFDD extraction for the acid treated sample. However, the absorption spectra of the supernatant and sediment for each centrifugation condition are almost identical, indicating no preferential removal of m-tube even at 48 000g. This result only changed slightly when the centrifugation force was increased to 105 000g. Under this condition, a strong M11

surface, but it also introduces negative charges on it, hence counterbalancing the positive charges arising from p-doping. We further studied the acid or base treatment effect by quantitatively neutralizing the acid-treated sample. This was done by titrating with a set of NaOH solutions with a concentration of 0.08, 0.2, 0.4, 0.8, 1.2, 2.0, and 10.0 M. In this process, 0.02 mL of NaOH solution was added to a suspension of acid-treated SWCNTs (20 mg) with PFDD (10 mg) in 25 mL of toluene followed by 1 h bath sonication to facilitate a thorough interaction between NaOH and the nanotubes. These titrations resulted in molar ratios of NaOH to SWCNT carbon (NOH/NC) of 0.000, 0.001, 0.0024, 0.0048, 0.0096, 0.0144, 0.024, and 0.12. After titration, the solvent was removed by centrifugation, and the SWCNT samples were extracted three times using 1:1 of PFDD:SWCNT ratio.4 UV spectra of the three successive extractions and Raman spectra of the third extraction for different NOH/NC ratios were collected and are displayed in Figure 2b,c and d. The UV spectra (Figure 2b) show that the background intensity increases with the NOH/NC ratio, indicating the selectivity of the extraction becomes poorer as more base is added. The M11 peak starts to appear when NOH/NC reached 0.0024, with the ϕ value reaching 0.36. This is the same level as the base-treated sample previously discussed. The Raman spectra in Figure 2c,d (more details in Figure S3) clearly show increased signal intensity in the metallic region (shadow area) with the increasing NOH/NC ratios, confirming the result from the UV spectroscopy measurement. Figure 3a summarizes the change of the ϕ value for three successive extractions as a function of NOH/NC ratio, and Figure 3b plots the overall yield and the average ϕ values as a function of the NOH/NC ratios for all three extractions combined. It can be seen in Figure 3a that the ϕ value decreases as a function of NOH/NC ratio and levels off at about 0.1, when the NOH/NC ratio reached 0.024 for the first extraction and 0.014 for the second and third extractions. This ϕ value is similar to that of the starting materials, indicating that the extraction process has completely lost its selectivity at this point. It should be noted that this titration is a heterogeneous reaction, and hence a prolonged reaction time is required to complete the reaction. In fact, after a drop of NaOH aqueous solution was added into the SWCNTs dispersion during titration, water dissolved into the solvent with NaOH precipitating as fine powder at the onset of sonication. As a result, the solution became cloudy but D

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Figure 4. (a) Calibration curve for the free polymer concentration measurement using photoluminescence (inset: linear region of the calibration curve). (b) Variation of the free polymer fraction (P/P0) as a function of total polymer concentration at PFDD/SWCNT ratios of 5/1 and 10/1 for both sc- and m-tubes (− − − line, a guide line of P/P0 = 0.138).

polymer wrapping density as a function of tube type and polymer-tube weight ratio. Therefore, sc- and m-SWCNT dispersions were prepared in toluene at PFDD/SWCNT weight ratios of 5/1 and 10/1. The sc-SWCNTs/PFDD dispersion with a sc-purity of about 99.9% was prepared from the PFDD extraction of the acid-treated SWCNT sample, and the m-SWCNT dispersion was prepared from a column chromatography purified sample with m-purity of 91% (see Figure S7). The 5/1 and 10/1 PFDD/SWCNT ratio samples were obtained by adding appropriate amounts of PFDD to the corresponding solutions. Figure S5 displays the solution photoluminescence (PL) spectrum excited at 385 nm of PFDD, mixtures of PFDD with sc-SWCNTs (5/1, w/w), and PFDD with m-WCNTs (5/1, w/w) in toluene. It shows these three solutions have almost identical PL curves, indicating that at the test concentration and polymer to nanotube ratio, there is always a large amount of polymer that exists in the solution as free chains, with no contact to nanotubes. The interaction between nanotubes in the highly pure sc-SWCNTs/PFDD solution enriched from the acid-treated sample was further investigated using photoluminescence excitation (PLE) mapping in the excitation wavelength range from 770 to 1020 nm, which covers all possible nanotube chiralities (n,m) in the plasma SWCNTs. The result is displayed in Figure S6a and shows well-resolved peaks for assigned chiralities, indicating no energy transfer occurs between the nanotubes. The obtained sc- and m-SWCNTs/PFDD solutions were then diluted by a factor of 2.5 to form a series of dilutions. The concentration of the PFDD (P0) and SWCNTs (C0) in the dispersions was determined by absorption spectroscopy.3 On the basis of the fact that PFDD fluorescence will be quenched by nanotubes upon contact or in close proximity, the concentration of free polymer (P) that is not interacting with nanotubes can be quantified by photoluminescence (PL) using a calibration curve as shown in Figure 4a, where a linear relationship between PL intensity and concentration can be observed up to 0.2 mg/L. Above this concentration, luminescence self-quenching of PFDD will occur and become significant beyond 1 mg/L. Therefore, to ensure a reliable concentration measurement, the polymer concentration of the original solution [P0] in our test was maintained below 5 mg/L, so that the free polymer concentration will be located in the

absorption band is still displayed in the supernatant, indicating the presence of a large amount of m-tubes. This centrifugal force (105 000g) is strong enough to remove bundles (see the Supporting Information), depending on factors such as bundle diameter, length, solvation, viscosity, the length of the centrifuge tubes, and the centrifuge times. Hence, a very low yield (0.5%) in the supernatant is obtained, and while this result is consistent with m-tubes being present in the solution as individual tubes, it does not preclude the possibility of having trace amounts of metallic tubes preferentially found in small bundles or even exclusively found in bundles. In addition, PLE mapping depicted in Figure S6 supports our hypothesis, yet cannot preclude the possibility of an undetectable amount of small bundles in the supernatant. Therefore, we can conclude that the acid and base treatment changes the surface condition of the nanotubes and thus alters their bundling behavior, which may be associated with a change in interactions with the solvent, wrapping polymers, and nanotubes themselves. The interaction of nanotubes with the conjugated polymer is an important factor.43−46 To shed further light on polymer/ tube interactions and its impact on selectivity, we need to consider the competition between nanotube bundling and dispersion. To achieve a selective dispersion, m- and sc-tubes are expected to have sufficiently different intertube interactions, while polymer−nanotube interactions may vary for m- and sctubes depending on the type of polymer used due to differences in polarizability (polar vs nonpolar polymer). These two competing interactions will determine nanotube dispersibility or colloidal stability in a given solvent. A strong nanotube− polymer interaction will result in a high polymer wrapping density and promote nanotube dispersibility, while a strong intertube interaction will strengthen nanotube binding in bundles, reduce nanotube dispersibility, and promote aggregation. A sufficient dispersibility difference between m- and sctubes is essential to achieve good enrichment selectivity. This can be obtained only when one of the following two conditions are satisfied: (1) polymer wrapping is specific to nanotube type while intertube interactions may not vary considerably; and (2) intertube interactions are specific to nanotube type while polymer wrapping may not vary considerably. To determine which condition is dominant during PFDD extraction, we designed an experiment that would provide insight on the E

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proton concentration or pH value on the SWCNT surface, and (3) the presence of a water layer on the SWCNT surface. The last factor is an essential condition to O2 doping on the nanotube surface.5 Fortunately, a water layer can be easily formed at the surface of carbon-based materials including carbon nanotubes, even diamond, under ambient conditions.15 With the presence of a water layer on the carbon nanotubes and a fixed oxygen concentration level in the air/solvent, the redox potential of the O2/H2O couple is determined by the pH value in the water layer on the nanotube surface. It is important to note that O2 doping under these mild conditions does not generate any apparent spectroscopic change, which is consistent with prior observations,42 yet based on the literature,8,17,41,42 it is the most likely contributor to redox processes at the SWCNT surface. Figure 5b compares the O2 redox potential at pH = 0,

linear region. Figure 4b plots the variation of free polymer fraction (P/P0) as a function of the total polymer concentration [P0]. It shows that P/P0 values remain essentially unchanged in a range between 0.12 and 0.15 for both m- and sc-tubes at both PFDD/SWCNT ratios of 5/1 and 10/1 in a broad PFDD concentration range (P0 = 0.05−5 mg/L). This result indicates that both m- and sc-tubes have a similar capacity to bind PFDD with about 86% of the polymer chains in solution interacting with the nanotubes. Furthermore, this fraction remained unchanged with the PFDD/SWCNT ratio going from 5/1 to 10/1 in this concentration range, indicating that the number of polymer chains in contact with a nanotube (nanotube−polymer complex) increases consistently with the PFDD/SWCNT ratio. This result is consistent with a loss of selectivity when high polymer ratios are used.3 Furthermore, this scenario could only manifest itself when the polymer chains are partially contacting the nanotubes, with a portion stretching out into solution. Coleman et al. derived a similar conclusion in their work on the basis of binding energy analyses for phenylenevinylene copolymers and SWCNTs.47 Therefore, we conclude that PFDD has a similar wrapping density on both m- and sc-tubes, and thus the data suggest that the wrapping polymer only provides a driving force for dispersing nanotubes in the solution, but in and of itself, it may not be responsible for the selectivity observed in PFDD extraction. It is important to note that the concentration range used for these experiments is lower than what is used for enrichment due to the PL selfquenching of the polymer at typical enrichment concentrations, yet we believe that the polymer wrapping density experiment provides some insight into what may be occurring at higher concentrations. We can rationalize these observations in that PFDD is a nonpolar polymer and has a dielectric constant similar to that of the solvent (3.0 for PFDD and 2.4 for toluene);48 hence the polymer/SWCNT interaction is nonpolar in nature and largely mitigated by the solvent. Similar results have been reported for other nonpolar adsorbates by molecular modeling.49,50 This will effectively negate polymer−SWCNT interactions driven by polarizing effects and thus result in similar polymer wrapping densities for both m- and sc-tubes (vide infra), even though the m-tubes have a much high polarizability.19−23 More importantly, a nonpolar aromatic solvent such as toluene or xylene will not interfere with the induced polar− polar interactions between nanotubes that may result from doping and are driven by polarizability. However, highly polar solvents would screen the m-SWCNTs from polar moieties, facilitate the nonselective dispersions of SWCNTs, and impede the rebundling/aggregation of m-SWCNTs necessary for scenrichment.51 Therefore, the enrichment selectivity of the acidtreated SWCNTs may be largely attributed to intertube interactions in the nonpolar solvent, toluene. The titration study described previously provides further evidence for our mechanistic understanding, in particular, how the surface acidity affects SWCNT doping and further enables a selective removal of m-SWCNTs driven by their high polarizability. The literature indicates that SWCNTs are easily p-doped by O2 under ambient conditions with moist air.7−10 The redox potential of O2 is determined by the O2/H2O redox couple as shown in eq 1.5,8,15 O2 + 4H+ + 4e− ⇌ 2H 2O

Figure 5. (a) Schematic illustration of the formation of polar−polar interaction between metallic (gray) and semiconducting (green) SWCNTs that are p-doped by O2/H2O redox process in humid air. (b) The comparison of electron energy range for the valence (V 1s) and conduction bands (C 1s) of the plasma tubes (shadow area) and the (10,9) chirality tube with the O2/H2O redox couple in humid air at different pH values on the nanotube surface.14,15

6, and 14 with the first valence band energy level (V 1s) of the nanotubes (shadow area for the plasma tubes).14,15 It is understood that charges can be added to or removed from both m- and sc-SWCNTs by the O2/H2O redox process, yet the following discussion will focus on sc-SWCNTs given that mSWCNTs have a constant density of states, and for moderate doping, the chemical potential will not reach the band edges, while the doping effect of sc-SWCNTs is expected to be more pronounced. It can be seen that V 1s is higher than the redox potential of O2 when the pH is below 6. Under neutral conditions, the nanotube surface has a pH value around 6.0, due to ambient CO2 dissolved in the surface water layer.15 Hence, SWCNTs can be easily p-doped by oxygen under these conditions, where most enrichment processes are conducted. However, V 1s becomes much lower than the O2 redox potential at a high pH value, for example, at pH = 14, and thus O2 redox processes are reversed and remove holes from SWCNTs. It is interesting to recall from the titration test that when NOH/NC reached 0.0024, the performance of the enrichment was the same as the base-treated sample first presented, and when NOH/NC reached 0.014, the enrichment completely lost its selectivity. These results correspond to two neutralization stages: neutralizing the defects related to COOH groups followed by removing holes by dedoping. This result implies that at 0.0024 NaOH equivalent, the acidic form of the COOH on the nanotube surface was completely converted to the basic form. Therefore, the concentration of COOH groups

(1)

This equation indicates three main factors that determine the potential to p-dope SWCNTs: (1) O2 concentration level, (2) F

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formation of m-SWCNT enriched precipitate in a dispersion. However, a high pH value, as obtained by base treatment of the SWCNTs, will reverse O2/H2O redox couple and conceivably dedope the nanotubes, resulting in a loss of selectivity due to a lack of intertube interactions, which are driven largely by polarizability. As such, those seeking to obtain high selectivity enrichment processes need to consider the nanotube process history, and the effect of acid/base chemistry in particular, leading up to the enrichment step of the process.

should be approximately 0.0024 or about 1 per 416 carbons. This value is lower than that reported previously (1−2%).52 This is easy to understand given that the nanotube samples in this reference were nitric acid-treated, which will generate more defects. After this point, the titration will gradually raise the surface pH and result in a decrease in p-doping. The complete loss of selectivity at NOH/NC = 0.0144 suggests that p-doping was completely removed at this point. This ratio equates to 1 equiv of hydroxide ion per 70 carbon atoms. Further considering the presence of COOH defects at 1 per 416 carbons, and subtracting its contribution to the titration (1/ 70−1/416 = 1/84), we obtain a doping level of 1 equiv to 84 carbons, which is at a similar level of estimated hole doping density for sc-SWCNT under ambient conditions (1/77).10 Having observed that polymer wrapping provides a similar driving force to disperse both m- and sc-nanotubes and that SWCNTs are susceptible to being p-doped and positively charged under neutral and acidic conditions, we ascribe the enrichment selectivity to the higher polarizability of mSWCNTs, which has been calculated to be about 103−105 larger than sc-SWCNT.19−23 This difference has been used to rationalize the mechanisms that underpin several sc-SWCNT enrichment techniques, such as CPE,4,5,53 dielectrophoresis,54−56 polymer-assisted aqueous two-phase partition,36,57 and chromatography.58−60 For enrichment using PFDD extraction in a nonpolar solvent, the higher polarizability of m-SWCNTs enables the generation of a strong induced polar− polar interaction with the p-doped SWCNTs as illustrated in Figure 5a. Although the m-tubes are also charged,17,61 the discussion above shows that sc-tube doping is feasible under ambient conditions where no efforts have been made to remove water and oxygen from the system. This interaction between the doped tubes and the more polarizable m-tubes significantly enhances the binding of the m-SWCNTs with other SWCNTs in bundles, and thus limits the dispersibility of m-SWCNTs (more difficult to exfoliate m-tube containing bundles) and also reduces their colloidal stability, leading to selective precipitation of m-tubes.3 Furthermore, this interaction must be occurring to achieve a very high sc-purity enrichment as the odds of m-tubes bundling with the doped tubes is greatly favored and consistent with experimental observations.3

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METHODS Materials. A raw plasma SWCNTs sample (purified RN000, Raymor Nanotech) and poly(9,9-di-n-dodecylfluorene) (PFDD) with a number-average molecular weight of 17.6 kDa and polydispersity index of 3.89 were used.3 Acid and Base Treatment of SWCNTs. 40 g of raw plasma SWCNTs sample was stirred in 1000 mL of 4 N HCl solution in a water/ethanol (3/7, v/v) mixture for 40 min, and was filtered and washed with water until the filtrate became neutral. About 1/10 of the solid was put aside for base treatment. The rest was subsequently washed with 500 mL of ethanol, acetone, and toluene (three times for each solvent). After filtration, the wet solid containing ∼95% toluene was stored as the acid-treated sample. Next, 1/10 of the solid was mixed with 100 mL of 0.5 M NaOH solution and stirred for 20 min, filtered and washed with water until neutral, then washed with organic solvents following the same procedure as the acidtreated sample to obtain the base-treated sample. The solids content of both the acid- and the base-treated samples were determined by weight analysis upon completely drying the sample under vacuum. Enrichment Based on PFDD Extraction. A SWCNTs sample (raw or acid-treated) was conditioned in a PFDD/ toluene solution (0.5/1 PFDD/SWCNTs weight ratio) to remove impurities and then was extracted using a 1/1 PFDD/ SWCNTs weight ratio to obtain the first supernatant. This extraction process was applied and repeated twice to the sediment from the previous extraction to obtain the second and third supernatants. Typically, to 25 mg (weight of dry sample) of a SWCNTs sample were added 12.5 mg of PFDD and 25 mL of toluene. The mixture was horn sonicated (mini-tip, cycle 60% and output 30%) for 30 min at ∼30 °C, and then centrifuged at 12 500 rpm using SS-34 rotor (RCF 18 690g) for 30 min to collect the sediment. The supernatant from this step was colorless and contained negligible SWCNTs. This process was then repeated to the sediment three times at a PFDD/ SWCNTs ratio of 1/1 in 25 mL of toluene for extractions. The supernatants from these three extractions were collected for characterization. Characterization. Absorption spectra were collected on a UV−vis−NIR spectrophotometer (Cary 5000, Varian) over a wavelength range from 300 to 2100 nm. A double beam mode was used with a pure solvent cuvette placed in the reference channel. The yield of the enrichment processes and the purity of the SWCNT materials were determined from the absorption spectra based on a previously reported method.3 Raman spectra were acquired with an InVia Raman microscope (Renishaw) from drop-cast samples. 514 nm (2.41 eV), 633 nm (1.96 eV), and 785 nm (1.58 eV) laser excitation sources and 50× magnification objective lens were used. Spectra were recorded from 100−3000 cm−1, with a resolution of 4 cm−1. For the polymer wrapping density studies, fluorescence spectra were recorded on a PerkinElmer LS 55 luminescence spectrometer,

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CONCLUSION The enrichment selectivity afforded by PFDD extraction is a result of a competition between polymer−tube and intertube interactions. In this process, PFDD possesses the proper combination of alkyl side chain length and main chain structure for the enrichment of sc-SWCNT in a specific chiral angle and diameter range of plasma torch derived nanotubes.42−44 PFDD wrapping simply provides a driving force to disperse nanotubes in toluene. Both m- and sc-tubes show a similar polymer wrapping density, indicating they have a similar van der Waals interaction with PFDD in this solvent, largely due to the solvent dielectric constant being similar to that of PFDD.46 It enables induced intertube interactions that drive m-tube aggregation with p-doped tubes, thus enabling a high selectivity. p-Doping by the O2/H2O redox process is postulated as the root cause for the driving force that leads to selective precipitation of m-tubes. This doping process is enhanced by increasing the surface acidity of nanotubes, which drives the O2/H2O redox couple forward. This effect not only leads to more stable m-tube containing bundles, which become less susceptible to exfoliation, but also a higher propensity for the G

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The Journal of Physical Chemistry C excited at 385 nm. The peak intensity at ∼420 nm was used for evaluating PFDD concentration.

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ASSOCIATED CONTENT

* Supporting Information S

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jpcc.6b05965. Investigation of star-like structures in the plasma SWCNT samples (SEM, TGA); Raman spectra of the enrichment products from the NaOH-titrated samples; ultracentrifugation study for NaOH-titrated PFDD/ SWCNT samples; photoluminescence investigation of PFDD/SWCNT solutions; and m-SWCNTs/PFDD polymer wrapping test using fluorescence spectroscopy (PDF)

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AUTHOR INFORMATION

Corresponding Authors

*Tel.: (613) 993-4456. E-mail: [email protected]. *Tel.: (613) 990-0705. E-mail: [email protected]. ca. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We appreciate the effort that Dr. Fuyong Chen has made in purifying m-tubes using chromatography. REFERENCES

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