Hydroxide Ions Stabilize Open Carbon Nanotubes in Degassed Water

Hydroxide Ions Stabilize Open Carbon Nanotubes in Degassed Water George Bepete,†,‡,∥ Nicolas Izard,§ Fernando Torres-Canas,†,‡ Alain Derré,†,‡ Arthur Sbardelotto,†,‡ Eric Anglaret,§ Alain Pénicaud,*,†,‡ and Carlos Drummond*,†,‡ †

CNRS, Centre de Recherche Paul Pascal (CRPP), UMR 5031, F-33600 Pessac, France Université Bordeaux, CRPP, UMR 5031, F-33600 Pessac, France § Université Montpellier, Laboratoire Charles Coulomb (L2C), UMR CNRS 5521, F-34000 Montpellier, France

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‡

S Supporting Information *

ABSTRACT: The main hurdle preventing the widespread use of single-walled carbon nanotubes remains the lack of methods with which to produce formulations of pristine, unshortened, unfunctionalized, individualized single-walled carbon nanotubes, thus preserving their extraordinary properties. In particular, sonication leads to shortening, which is detrimental to percolation properties (electrical, thermal, mechanical, etc.). Using reductive dissolution and transfer into degassed water, open-ended, water-filled nanotubes can be dispersed as individualized nanotubes in water−dimethyl sulfoxide mixtures, avoiding the use of sonication and surfactant. Closed nanotubes, however, aggregate immediately upon contact with water. Photoluminescence and absorption spectroscopy both point out a very high degree of individualization while retaining lengths of several microns. The resulting transparent conducting films are 1 order of magnitude more conductive than surfactant-based blanks at equal transmittance. KEYWORDS: carbon nanotubes, aqueous dispersions, photoluminescence, Raman, eau de nanotubes, hydrophobic interaction, nanotubide

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agents preventing reaggregation have been used, followed by ultracentrifugation to produce individualized nanotube dispersions.8,18−22 Using such dispersions, Krupke et al. were able to sort carbon nanotubes into metallic and semiconducting tubes by dielectrophoresis.9 Later, density-gradient centrifugation23 was used to separate carbon nanotubes, and in 2013, SWCNTs were also separated by their electronic character in a biphasic aqueous system.24 In all those methods, the use of sonication imposes severe limitations on the structural integrity of the material as they shorten the nanotubes to mostly less than 200 nm.25,26 Furthermore, dispersant molecules need to be rinsed away from the nanotubes after deposition. Alternatively, chemically modified SWCNTs can be efficiently dispersed in water as long individualized nanotubes, but they exhibit diminished electronic and optical properties caused by the covalent functionalization of the nanotube sidewalls.27 The main hurdle to the widespread use of SWCNTs remains the lack of methods to produce pristine, unshortened, unfunctionalized, individualized SWCNTs, thus preserving their extra-

hysical properties of single-walled carbon nanotubes (SWCNTs) such as high mechanical strength, high electronic mobility, chemical inertness, lightness, and high aspect ratio render them suitable for potential applications in, e.g., multifunctional composites or organic electronics.1−7 As-synthesized SWCNTs are intrinsically bundled due to the high attractive van der Waals (vdW) forces between adjacent tubes (0.5 eV/nm).8 The SWCNT bundles consist of a mixture of semiconducting and metallic nanotubes that renders all the bundles metallic in nature.9−11 However, the mostpromising applications of SWCNTs in transparent conducting electrodes,12,13 biosensors,14,15 nanoelectronics,16 (opto)electronic systems, DNA sequencing, spintronics, and electrochemical energy storage require their prior solution-phase disentanglement into individualized SWCNT dispersions.17 Individualization has been performed as early as 2002 by sonication and ultracentrifugation and reported in a seminal article by Smalley’s group.18 Dispersions of individualized, surfactant-coated tubes showed structured optical transitions for all tubes and photoluminescence for semiconducting tubes, both features that have been associated since then with individual, rather than bundled, tubes. Together with sonication in organic solvents or water, a number of stabilizing © XXXX American Chemical Society

Received: June 7, 2018 Accepted: August 8, 2018 Published: August 8, 2018 A

DOI: 10.1021/acsnano.8b04341 ACS Nano XXXX, XXX, XXX−XXX

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Figure 1. (Meta)stable dispersions are prepared with opened tubes and degassed water. Pictures of the different dispersions of SWCNTs in water samples. (a) Reaggregation in an unstable dispersion of open-ended SWCNTs in normal water (not degassed) (b). Homogenous dispersion of individualized open-ended SWCNTs in degassed water. (c) An aggregated dispersion of closed SWCNTs in degassed water.

ordinary properties. In that direction, Wang et al. were able to produce sodium deoxycholate stabilized full length SWCNTs in water by utilizing a superacid-to-surfactant exchange method.28 Since 2005, it has been shown that reductive dissolution of nanotubes yields solutions of individualized, unshortened, reduced carbon nanotubes,29,30 also known as nanotubides.31 Likewise, graphenide solutions have also been obtained.32−34 However, their use has not widely spread due to their air sensitivity. We have recently shown that (meta)stable dispersions of single-layer graphene (SLG) in water can be produced by transferring fully exfoliated graphenide solutions in tetrahydrofuran34 into previously degassed water with no additives.35−37 The remarkable stability of graphene in degassed water is due to the adsorption of hydroxide (OH−) ions onto graphene, leading to a colloidal dispersion of graphene bearing negative ionic charges.35 We report here that, using a similar method, SWCNTs can be individualized and dispersed in a water−dimethyl sulfoxide (DMSO) mixture, without surfactants nor sonication, provided that they are open-ended. Absorption, photoluminescence (PL), and Raman spectroscopic signatures all show that the nanotubes are individualized. A crucial factor appears to be the opening of the tubes that allows them to be filled with water and, hence, reduce the dielectric contrast with their environment. The stability of a dispersion of SWCNTs is determined by the interplay between attractive and repulsive forces between individual tubes. A significant destabilizing factor is constituted by long-range dispersion (van der Waals) forces, which are determined by the dielectric contrast between the SWCNTs and the solvent, which is likely to be lower for opened (waterfilled) than for closed (vacuum-filled) tubes. Consequently, vdW forces will be diminished (and the stability of the suspension augmented) for opened versus closed SWCNTs.

SWCNTS) are closed-ended, whereas purified single-walled carbon nanotubes (P2-SWCNTs) are open-ended.41 It should be stressed that purification, in theory, also shortens the nanotubes albeit less than sonication [we still observe long (4 and 8 μm) nanotubes at the end of our process (Figure S5)]. The P2-SWCNTs are obtained (by the supplier) by air oxidation and subsequent acid treatment purification of APSWCNTs. AP-SWCNTs and P2-SWCNTs both exhibit low and similar functionality (see the details on X-ray photoelectron spectroscopy, or XPS, characterization in Figure S1 and Table S1). D-band analysis in Raman spectroscopy shows virtually no difference between the two types of tubes (ID/IG values are 0.02 and 0.03, respectively, for AP- and P2SWCNTs at 785 nm), confirming that P2-SWCNTs have not been functionalized during purification (Figure S2). Dispersibility of P2-SWCNTs and AP-SWCNTs in bile salt (and other surfactants) aqueous dispersions and in organic solvents such as dimethylformamide (DMF) is similar. The air oxidation and subsequent acid treatment step is known to result in openended SWCNTs. We checked the opening, or not, of the tubes by preparing peapods42 and imaging the samples by transmission electron microscopy (TEM). AP-SWCNTs remained empty, whereas P2-SWCNTs were all filled with fullerenes (Figure S3). In the following, P2-SWCNTs and AP-SWCNTs tubes are described as “opened” or “closed”, respectively. Nanotubide Solutions of Individualized Tubes. SWCNTs (both types) were first individualized by reductive dissolution in DMSO:29,30,43−48 a potassium salt of nanotubes was prepared by reacting K under an inert atmosphere on a hot plate with SWCNT powder, as has been reported for graphite intercalation.37,45,49 Stoichiometry was chosen as KC25 to have a sufficiently high concentration46 but low functionalization upon later exposure to water.50,51 Nanotubide solutions were obtained by exposing KC25 to DMSO for 4 days with stirring and no sonication and then centrifuging (4000 rpm for an hour) to remove undissolved tubes and catalytic material.52 Surfactant-Free Aqueous Dispersion (Eau de Nanotube). Nanotubide solutions were then removed from the inert atmosphere glovebox and rapidly injected into slightly basic water (pH 9). A total of three parallel experiments were performed: (i) opened tubes in normal (not degassed) water, (ii) opened tubes in degassed water, and (iii) closed tubes in degassed water. Experiments i and iii failed, i.e., aggregation of tubes occurred within a minute after injection of the DMSO nanotubide solution into water (Figure 1a,c). Experiment ii

RESULTS Closed (Empty) vs Opened (Filled) Tubes. Carbon nanotubes are opened and water-filled by tip sonication, a mostly unnoticed fact prior to the works of Cambré and Wenseleers,38−40 but sonication had to be discarded because non-shortened tubes are required. Closed and empty SWCNTs were obtained from Carbon Solutions, Inc. They have been synthesized by the electric arc technique and have a narrow diameter distribution peaking at around 1.4 nm [data of the supplier, consistent with absorption, PL and Raman data (vide infra)]. As-prepared single walled carbon nanotubes (APB


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Figure 2. Spectroscopic signatures of eau de nanotubes (EdN) compared to bile salt dispersions (BS-disp). (a) Absorption spectra (normalized to the absorbance at 660 nm), (b) photoluminescence map of EdN and (c) BS-disp, (d) Raman spectra of EdN after subtraction of the DMSO−water signal (remnant DMSO peak at 1420 cm−1 is marked with an asterisk), and (e) BS-disp, normalized to the maxima of the RBM and the G+ band intensity at low and high frequencies, respectively.

spectroscopies were recorded and compared for both samples (EdN and BS-disp). Absorption. The electronic structure of SWCNTs can be monitored by absorption spectroscopy:27,47 absorption bands correspond to excitonic transitions associated with mirrored van Hove singularities.54,55 For a typical nonselective synthesis of SWCNTs by the electric arc method, leading to a distribution of diameters between 1.2 and 1.5 nm, absorption peaks in the ranges of 1400−1800, 800−1200, and 400−600 nm are assigned to excitonic transitions associated, respectively, to the first (S11), second (S22), and third (S33) pairs of van Hove singularities of semiconducting nanotubes, whereas absorption peaks in the range 600−800 nm are assigned to the first transition of metallic nanotubes (M11).56,57 When SWCNTs are assembled into bundles, the lifetime of excited excitonic states decreases due to exciton energy transfer (EET),58 and the width of the absorption peaks increases accordingly18,56 In contrast, the absorption spectrum of individualized SWCNTs displays series of well-resolved fine spectral structures, corresponding to sharp excitonic transitions of different and characteristic energy for each tube diameter18,56 The UV−vis-NIR spectra of EdN and BS-disp are compared in Figure 2a. The two spectra are very similar, showing a series of well-resolved structures between 400 and 1200 nm (no S11 spectral signatures of SWCNT can be measured in aqueous dispersions above 1350 nm due to the strong absorption of water), typical of samples containing a majority of individualized SWCNTs. No significant shifts are observed on S33 and M11 and only weak shifts are observed (less than 5 nm) on S22 transitions, indicating that the

yielded a homogeneous suspension of carbon nanotubes in water (Figure 1b). To distinguish this sonication and surfactant-free dispersion from the classical surfactant- and sonication-aided SWCNT dispersions, we name it as “eau de nanotube” (EdN). The carbon nanotube concentration in EdN was determined by dry extract as 26 mg.l−1 (after subtraction of the KOH fraction, assuming one KOH per 25 carbon atoms). Additionally, the stability of EdN was tested against pH variations when nanotubide solutions were injected into degassed water of varying pH (by prior dissolution of KOH): 5.6, 9, 10, 12, and 14. pH 12 and 14 resulted in immediate reaggregation, whereas EdN prepared with pH 5.6, 9, and 10 degassed water were stable. The most stable dispersion was prepared by mixing the nanotubide solution with degassed water with a pH of 9. It should be emphasized that AP tubes contain more catalyst impurities than P2-tubes (ca. 37% against 12%, see the Methods section) and are, therefore, more charged per carbon. However, control experiments show that P2 nanotubides with higher potassium content (up to KC8) are also dispersible in water. Evidence of the Individualization of Tubes. A dispersion of open-ended carbon nanotubes with bile salt was prepared, tip-sonicated, and ultracentrifuged to have a reference sample of individualized tubes. This additional sample was called BS-disp for bile-salt dispersion.38,53 The concentration in carbon nanotubes in BS-disp was determined by dry extract as 260 mg·L−1 (bile-salt proportion was measured by thermogravimetric analysis, TGA, and subtracted). Absorption, Raman, and photoluminescence (PL) C


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Figure 3. (a) Picture of a transparent film of SWCNT fabricated by ultrasonic spray coating of EdN on a glass substrate. (b) Atomic-force microscopy micrograph of a SWCNT film on a glass substrate. Scale bar: 4 μm. (c) Transmittance vs sheet resistance comparisons of transparent conducting films (TCFs) made from EdN and from BS-stabilized SWCNT dispersions together with data from the literature. EdN1a, as-made spray coated films; EdN2a, as-made vacuum filtrated films; EdN1b, annealed spray-coated films; and EdN2b, annealed vacuum filtrated films. Data included from other sources and marked as Parekh,76 Wang1,77 Hecht,75 Mirri,71 and Wang2.28

have expected far less PL intensity for individualized SWCNTs in EdN than for the surfactant wrapped SWCNTs in BS-disp. Thus, the somewhat-high PL intensity in EdN is another indication that the individualization of SWCNTs is at least as good in EdN as in BS-disp. Finally, the relative intensity is larger for the peak at large emission wavelength (1588 nm, IBS‑disp/IEdN = 3.6) for BS-disp and at small emission wavelength (1540 nm, IBS‑disp/IEdN = 1.8) for EdN, suggesting less efficient EET for EdN with respect to BS-disp, which supports, again, a better individualization of the nanotubes. Raman Spectroscopy. Figure 2d,e compare the Raman spectra of EdN and BS-disp for two laser exciting lines, in the spectral ranges of the radial breathing modes (RBM, 140−200 cm−1) and the D and G bands (1300−1350 and 1500−1600 cm−1, respectively), normalized to the maxima of the RBM and G band intensities, respectively. The Raman signals of DMSO and water superimpose to that of the SWCNT, but only a weak DMSO peak can be observed in these spectral ranges (around 1420 cm−1, marked by stars in Figure 2d) and the relative intensity of the bending mode of water (expected at 1645 cm−1) is too weak to be detected. Only resonant SWCNTs, i.e. those having an excitonic transition within an energy window close (typically closer than 0.1 eV) to the energy of incident or scattered photons, contribute significantly to the spectra.62 For RBM, the energy of incident and scattered photons are very close so that resonance occurs only for a limited type of nanotubes. Therefore, the use of several laser lines is particularly important in Raman studies of SWCNTs to get a better sampling over various diameters and chiral angles. For SWCNT diameters in the range of 1.2−1.5 nm, the spectra excited at 638 nm (1.94 eV) and 785 nm (1.56 eV) are assigned to metallic nanotubes (M11 transition) and to both small semiconducting (S22 transition) and large metallic (M11 transition) nanotubes, respectively.63 These assignments are confirmed by the profiles of the G band (Figure 2d,e): (i) at 638 nm, the G band displays for the two samples one broad and asymmetric line and one Lorentzian corresponding to the expected signatures for strong electron-LO coupling and TO for metallic nanotubes, respectively;64 and (ii) at 785 nm, the profiles of the G band are significantly different, with an intense and broad profile of the G− band (i.e. of the lowfrequency component of the G bunch) for the EdN characteristic of metallic nanotubes,64,65 which is much weaker for BS-disp, which is tentatively assigned to a narrowing or a shift of the resonance window, as supported by the UV−vis− near-infrared (NIR)spectra. Accordingly, the RBM profiles are

dielectric environments around the nanotubes are not very different. For instance, energy red shifts of S11 and S22 transitions in the range 10−20 meV (corresponding to wavelength shifts of about 10 to 30 nm) are measured between closed (non-ultrasonicated and therefore unfilled) and open-ended (ultrasonicated and, therefore, water-filled) nanotubes.38,40 Red shifts of several tens of millielectrovolts (several tens of nanometers) are also observed between isolated nanotubes and bundles, typically 50 meV/80 nm for S11 and 20 meV/20 nm for S22 for HiPCO tubes.18 Therefore, the similarities between the two spectra support that SWCNTs in EdN are water-filled and mainly dispersed as individual carbon nanotubes.40 Photoluminescence. Figure 2b,c compare the photoluminescence excitation (PLE) maps of EdN and BS-disp, respectively (the PLE intensities were normalized by the corresponding absorbance for the two samples). Interactions between semiconducting and metallic nanotubes in bundles provide nonradiative relaxation channels for excitons through EET, which quench the PL signal.18 Therefore, the measurement of a PL signal indicates that nanotubes are individual or assembled into very small bundles (not containing metallic tubes), as observed in BS-disp. A total of three main peaks are observed for BS-disp at excitation and emission wavelengths of about 855 nm/1540 nm, 936 nm/1540 nm, and 938 nm/1588 nm, plus the onset of another peak around 1000 nm/1580 nm. From the pioneer measurements by Bachilo et al. on sodium dodecyl sulfate (SDS)-wrapped nanotubes,59 these peaks are tentatively assigned to (11,7) and/or (10,9), to (12,7), to (11,9) and to (15,4) nanotubes, respectively. The peak at high excitation energy (855 nm/1540 nm) is unexpectedly not observed in EdN. This could be due to a diameter-selective efficiency of the dispersion method. However, the two peaks at low excitation energies are observed on EdN at 944 nm/1540 nm and 954 nm/1588 nm with a slight red shift in S22 excitation (and no shift in S11 emission), and the onset of another peak is also observed around 1000 nm/1580 nm. When normalized by the absorbance, as in Figure 2b,c, PL intensity in EdN is ca. half of that of BS-disp. This is noteworthy because bile salts are known to be some of the best surfactant to disperse SWCNT, maximizing the distance between PL quenching sites and therefore optimizing their PL signal,60 while water by itself is an unfavorable medium for PL, as shown by Hertel et al.61 For example, the PL intensity of SWCNT drops by a factor of 5 when surfactants are exchanged by water in their close environment.61 Therefore, one would D


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DISCUSSION Schäfer et al. have hydrogenated graphene using a Birch-type reaction in which graphene was first reduced to graphenide by lithium in liquid ammonia and then water was added, at −78 °C, to the mixture.50 The authors hypothesized that there were single-electron transfers from graphenide to water forming H radicals. Those radicals, in turn, could then either react together to form molecular hydrogen or hydrogenate graphene. In their experiment, 2 equiv of water per metal atom were added to liquid NH3 at −78 °C and were immediately frozen and only slowly released when the temperature was increased, favoring low local concentrations of H· radicals and, hence, graphene hydrogenation rather than H2 production.50 Here, in contrast, the nanotubide solution in DMSO is rapidly injected in a large amount of water. Because DMSO and water mixing is an exothermic process,78 the temperature rose to 36 °C. A small amount of bubbles can be observed upon mixing the nanotubide solution with water, pointing to molecular hydrogen formation rather than nanotube hydrogenation. Furthermore, no increase in the D band has been observed upon reaction with water. Raw nanotubes and dried films show the same ID/IG ratio, and the film conductivity does not increase upon annealing (Figure 3c), contrary to graphene films from graphene water,35 in which defects are removed by annealing. Hence, a chemical sequence in agreement with the experimental observations is:

relatively close for the two samples, and the main difference is the much larger relative intensity of the high-frequency component (around 200 cm−1) for BS-disp at 638 nm, which supports as well a slightly narrower resonance window in EdN, leading to a more-drastic selectivity of the excited nanotubes in Raman. However, differences in the RBM profiles might be due to the diameter selectivity of the dissolution process,66 supported by the non-observation of the PL peak at 855 nm/1540 nm. Extended studies using more-exciting laser lines in Raman and broader excitation and emission spectral ranges in PLE will be required to settle this point. In short, absorption, PLE and Raman results all support that the individualization is comparable or even better in EdN than in BS-disp samples. This is evidenced by narrow absorption peaks and quite intense and narrow PL peaks as well as narrow Raman resonances. Defects. The D band is a second-order double-resonant mode involving elastic scattering on structural defects.67 Therefore, the D-to-G intensity ratio is widely used in the literature to estimate the amount of defects on graphitic materials including carbon nanotubes.45,68,69 The D-to-G intensity ratio is small for EdN (0.05, Figure 2d) and films made from EdN (0.04, see below) and close to that of the corresponding powder (0.03, Figure S2, values compared at 785 nm), which indicates that no significant damage or functionalization has occurred during the transfer of carbon nanotubide solution to water. Furthermore, EdN shows for both exciting wavelengths a smaller ID-to-IG ratio than BS-disp (Figure 2d,e), exemplifying a milder individualization process. Films. Transparent conducting films70,71 (TCFs) of opened SWCNTs were prepared using the vacuum filtration method or ultrasonic spray-coating of EdN on glass substrates (Figure 3a,b) and PET substrates. Films made by both techniques showed equivalent properties (Figure 3c). Films were also prepared with BS-disp for comparison. Both films were extensively rinsed with water to remove KOH for EdN films and bile salt in BS-disp films. At equivalent transmittance, TCFs made from EdN have close to 1 order of magnitude better conductivity than films made from BS-disp (Figure 3c), as had been already seen for nanotubide films versus surfactant-based films.72,73 The very close electrical performances obtained for as-deposited films and annealed films made from EdN (Figure 3c) confirm that the preparation technique does not produce functionalized SWCNTs, consistent with the Raman defect analysis (Figure 2). Thicker films (T < 90%) are less conductive after annealing at 500 °C than thinner films (T > 90%). We hypothesize that this increase in sheet resistance for thicker films could be due to the reaction between trapped KOH and carbon nanotubes, known to occur around 400 °C.74 Because all films are rinsed with water, this would be lesseffective for thinner films in which KOH is washed away more efficiently. Finally, in Figure 3c, it can be seen that other films from the literature are more conductive than those from EdN. We were actually limited in our choice of SWCNTs because we needed a source of rigorously closed tubes that could be later opened. However, the examples by Hecht et al.75 and Mirri et al.71 on super-long tubes dissolved in super-acids, unshortened by sonication, lead us to believe that the EdN procedure has the potential to produce highly performing films from benign, degassed water suspensions.

C.25−K+ + H 2O − >C025 + K+OH− + H.

(1)

where hydrogen radicals react together to form H2. No sonication has been used in the process; hence, nanotubes have not been shortened. Indeed, long entangled tubes can be seen on the AFM image of Figure 3b. Furthermore, backside absorbing layer microscopy (BALM)79 allowed us to image 4 and 8 μm long nanotubes that could not have withstood sonication (Figure S5). When water is in contact with a hydrophobic surface, dissolved gases adsorb at the interface in the form of small nanometric-size bubbles (nanobubbles) or form a low-density (depleted) layer. Avoiding the formation of this layer by removing the dissolved gases from the aqueous phase has significant implications in the interaction between hydrocarbon surfaces, reducing the range and strength of the apparent hydrophobic interaction. This fact has been clearly evidenced by direct surface forces measurements,80,81 and it has important implications on the stability of hydrophobic colloids.82 The rapid injection of the open-ended SWCNT nanotubide solution in DMSO into degassed water is accompanied by a high degree of agitation, ensuring that individualized nanotubes are instantaneously mixed with water, causing hydroxide anion adsorption (as typically happens when a hydrophobic surface is in contact with water).83,84 As in the case of graphene,35 SWCNTs become then electrically charged in degassed water as a consequence of the spontaneous adsorption on their surface of OH− ions, further confirmed by their electrophoretic mobility (see below). As two nanotubes come together, they experience a repulsive force because of the overlap of their associated counter-ion clouds. Thus, the instability of EdN at pH > 10 is due to the increased screening of the electrostatic repulsion when the ionic strength of the system is augmented. EdN will be most stable when the optimum balance between surface charge density (enhanced E


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Figure 4. (a) Probability distribution of measured electrophoretic mobility of the SWCNTs in EdN. (b) Interaction between parallel graphene-like cylinders versus surface−surface distance D calculated as described in the text and the Supporting Information. Salt concentration: 5 mM. Cylinder length: 1 μm. Diameter: 1.5 nm. T: 298 K.

by OH− adsorption) and electrostatic screening (minimized by reducing the total concentration of charged species) is achieved. Elaborating a useful description of the interaction between SWCNTs in water is a challenging problem. First, we are not dealing with a well-established material: samples of SWCNTs are intrinsically heterogeneous. The properties of individual tubes (and in particular their dielectric characteristics) will depend on their chirality, size, and metallic or semiconductor character. Second, SWCNTs are anisometric; for this reason, the interaction between individual tubes will depend on their relative orientation. We will only discuss here the interaction between parallel tubes (in a per-length basis), which appears to be the worst case scenario for the flocculation and destabilization of dispersed SWCNTs. For the case of SWCNTs in water, one should consider the interplay between stabilizing electrostatic forces and the attractive dispersive and hydrophobic interactions, in much the same spirit as we recently discussed the interaction between graphene flakes in this solvent.35 SWCNTs in EdN are negatively charged, as determined from electrophoretical measurements using direct particle tracking. A typical experimental distribution of electrophoretical mobility in the EdN is presented in Figure 4a. The measured mobility depends on the (random) relative orientation between the applied field and the SWCNTs. We have used the approximation proposed in ref 85 to estimate the value of the ζ potential (−67 mV) from the measured electrophoretic mobility μav values (a description of the data treatment is presented in the Supporting Information). We have taken the value of ζ calculated in this way as a conservative approximation of the SWCNTs surface potential to calculate the electrostatic repulsive interaction between SWCNTs. As mentioned before, the destabilizing van der Waals forces between suspended SWCNTs are a function of the dielectric contrast between the nanotubes and the suspending media. It is customary to factor this interaction energy in a term determined by the nature of the interacting interfaces (object− medium−object), the Hamaker coefficient AH, and a second term, a shape factor, determined by the geometry of the problem (shape and separation between the interacting bodies). Rajter et al. have demonstrated the significant reduction in AH for SWCNTs interacting through water (compared to vacuum) due to the reduced dielectric contrast

in the solvent. Most-relevant for the present study, they illustrate how partially or fully filling SWCNTs with water have significant consequences for the value of AH.86 Full description of the calculation of the interaction energy between two empty or filled SWCNTs in water and of the approximations involved are given in the Supporting Information, while the result, a rough estimate of the SWCNT−SWCNT interaction calculated by adding the electrostatic repulsion and the vdW attraction, is presented in Figure 4b. The experimental conditions considered are enumerated in the caption. As can be observed, a repulsive energy barrier limits the approach of the cylinders, stabilizing the aqueous dispersion. As a consequence of the reduced vdW attractive force, this barrier is enhanced for the filled cylinders (opened SWCNTs). For the particular dimensions of the tubes chosen (length of 1 μm and diameter of 1.5 nm), an increment in the energy barrier of the order of 17 kBT is obtained after filling the SWCNT with water. Because the probability of overcoming the barrier decreases exponentially with its size, even a modest reduction in its magnitude will have a central impact on the stability of the system. This difference acquires more relevance as the destabilizing attractive hydrophobic attraction has not been included in this calculation. The hydrophobic interaction is detrimental for the stability of SWCNTs in water. However, its range and strength can be significantly reduced by degassing the water.81 This effect has proven to be useful to stabilize hydrocarbon emulsions82 and graphene suspensions.35 Obviously, the assumptions introduced to estimate the SWCNT interaction energy limit the quantitative validity of these results, although they provide an indication of the influence of filling the SWCNTs on their interaction. In particular, we have assumed that the properties of water filling the SWCNTs are identical to those of bulk water. In fact, it has been widely documented that severe confinement, limitation on the hydrogen bonding and interaction with SWCNT walls change the phase boundaries.87−90 Thus, a more-realistic description that considers the nature of the SWCNTs and the real properties of confined water is required for an accurate description of the system under study.

CONCLUSIONS 20 years after the first carbon nanotube synthesis,91,92 carbon nanotubes can now be dispersed under ambient atmosphere as single individual objects without having been shortened or coated. EdN opens perspectives for organic, printable F


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at pH 9, adjusted by adding KOH. Nanotubide air exposure was kept as minimal as possible before mixing with water. The vial containing the SWCNTs dispersion in the DMSO-degassed water mixture was tightly capped and stored at room temperature or in a fridge at between 3 and 6 °C for further use and characterization. In a typical experiment, 2.5 mL of a 0.1 mg/mL DMSO carbon nanotubide solution was rapidly injected into a glass vial containing 17.5 mL of previously degassed water to form a 20 mL dispersion containing a 0.0125 mg/mL of SWCNTs in a DMSO-degassed water mixture, 87.5% water, and 12.5% DMSO by volume. For degassing, the water was subjected to agitation with a rotating Teflon stirrer under a reduced pressure of 0.2 mbar for 30 min. Next, the air pressure was gently increased back to atmospheric pressure. Bile-Salt Dispersion, BS-Disp. Dispersions were made with 0.3% SWCNT (9.1 mg) and 0.6% bile salt (18 mg, Fluka 48305, 50:50 wt % of sodium cholate and sodium deoxycholate) in 3 mL of freshly deionized water. The mixture was tip-sonicated for 30 min at 20% power and 50% pulses on a Branson Sonifier 250 ultrasonicator. The resulting dispersion was diluted with 18 mL of a 0.6% bile salt dispersion resulting in 9.1 mg of SWCNTs, 126 mg of bile salt, and 21 g of water. This diluted dispersion was ultracentrifuged at 150000g for 30 min in a Sorvall WX Ultra Series centrifuge from ThermoScientific. After separation of the centrifugate, the supernatant was centrifuged again at 150000g for 90 min. TGA measurements of the dry extract showed a content of 5% SWCNTs and 95% bile salt. Absorption Spectroscopy. Absorption spectra were recorded in 10 mm optical path quartz cuvettes using a UV−vis−NIR spectrometer (JascoV-670) in the wavelength range of 380−1350 nm. A baseline correction was performed using a DMSO and water mixture for EdN and BS solution for BS-disp. Photoluminescence Excitation Experiments. Photoluminescence spectroscopy was performed using an home-built micro-PL setup. The output of a titanium−sapphire laser (excitation wavelength ranging from 800 to 990 nm) was focused onto the sample (inside a quartz cell, near the entrance quartz windows) with an infrared microscope objective (40×, 0.45 NA). PL was collected by the same objective (in back-scattering geometry), and the direct laser line was rejected by a long-pass filter. PL was recorded with a 500 mm spectrometer and 150 lines/mm grating, and detection was performed by a nitrogen-cooled 1024 pixel linear InGaAs array. Excitation power was continuously recorded to normalize PLE intensities. No correction for reabsorption was performed (absorption is dominated by water and comparable for the two samples). Raman Spectroscopy. Raman spectroscopy was performed on an Xplora spectrometer from Horiba-Jobin-Yvon and an Invia spectrometer from Renishaw at different excitation wavelengths using a macrosample holder that contained a cuvette filled with EdN (1 cm pathway). The peak positions were calibrated using the T2g peak of silicon (520.5 cm−1) and the G band of highly ordered pyrolytic graphite (1582 cm−1). Films and Electrical Measurements. Transparent conducting films on glass and PET substrates were made by (i) vacuum filtration followed by stamping and (ii) by ultrasonic spraying of the SWCNT in water dispersion. Both techniques gave identical results in terms of transparency-to-conductivity ratio (cf. Figure 3c). Sheet resistance measurements were done using a four-point probe station. Films by Spray. Films were made using an ultrasonic spray robot. The glass substrates were fixed onto a hot plate at 90 °C during spraying to evaporate the solvent immediately after coating. The films were rinsed by washing in successive water baths to remove KOH and surfactant. Films by Filtration. Using vacuum filtration, different volumes of EdN/BS-Disp (0,5, 1.0, 1.5 mL,...) were diluted into 50 mL of distilled water and filtered through a 100 nm pore size nitrocellulose membrane filter (Millipore). A further 250 mL of distilled water was filtered through the nitrocellulose membrane to wash off the KOH and bile salts off of the nanotubes. The SWCNT films were deposited on carefully cleaned glass substrates by stamping. The nitrocellulose membrane was dissolved by placing the films successively in several acetone, methanol, and water baths for 15 min in each bath.

electronics, sensors, and other devices. TCFs from EdN have been demonstrated. Potent SWCNT sorting capabilities by electronic character and indices on (shortened) surfactantbased aqueous dispersions have been demonstrated.24,93 Preliminary experiments show that EdN can be prepared with surfactant containing water (at higher concentrations), thus opening the way to such sorting on long SWCNTs. Furthermore, EdN could be used to carry individual SWCNTs across cell membranes. Finally, EdN brings additional experimental evidence as to the role of OH− ion as stabilizing agents.94 Although not as stable as graphene water, EdN has a shelf life of a few weeks or months under refrigeration at 3−6 °C.

METHODS Materials. Studies were carried out using AP-SWCNTs and P2SWCNTs prepared by the electric arc discharge method using a nickel and yttrium catalyst (Carbon Solutions Inc.). The P2-SWCNTs are produced from the purification (by the supplier) of the AP-SWCNTs by mild air oxidation and subsequently treated in concentrated HCl followed by washing with copious amounts of water to remove the catalyst and acid. The purified material closely approximates the pristine state with low functionality and low chemical doping. Dispersibility is similar to that of the AP-SWCNTs (info from Carbon Solutions). Both AP-SWCNTs and P2-SWCNTs contain no sidewall functional groups and are not dispersible in water or in DMSO. APSWCNTs and P2-SWCNTs batches we used contained, respectively, ca. 37% and 12% non-carbon (metal catalyst) impurities (see Figure S4) that are known not to dissolve during reductive dissolution and are separated during the mild centrifugation step.52 The AP-SWCNTs are capped (closed ends), while the P2-SWCNTs are open-ended. Transmission electron microscopy of SWCNTs that have undergone peapod transformation (heated in the presence of fullerene) was used to confirm that P2-SWCNTs are open-ended, while AP-SWCNTs are capped. The P2-SWCNTs were completely transformed to peapods (filled with fullerene molecules), while the AP-SWCNTs were not transformed to peapods (Figure S2). Reductive Dissolution of SWCNTs. SWCNT salt (nanotubides) preparation and dissolution experiments were carried out inside an argon-filled glovebox environment. In a typical reaction, 27 mg of freshly cut potassium metal (K) was added to a 20 mL glass vial containing a 207 mg SWCNT sample. The mixture of K and SWCNT sample contained in a glass vial was heated on a hot plate at 180 °C with continuous mixing for 10 min using a stainless-steel spatula. The vial was left on the hot plate, and the mixture was mixed with the stainless-steel spatula at every 10 min interval for 1 h and then every 30 min for 4 h. The product was allowed to cool to room temperature, and the vial was tightly capped for later use. The salts were labeled AP-SWCNTs_KC25 and P2-SWCNTs_KC25 for APSWCNTs and P2-SWCNTs, respectively. Inside the glovebox, 4 mg of P2-SWCNTs_KC25 salt (or AP-SWCNTs_KC25 salt) was dispersed in 20 mL of dry analytical-grade DMSO solvent (or 0.2 mg/mL). The carbon nanotubides dissolved immediately and formed a black dispersion in DMSO. This solution was tightly capped and sealed with parafilm and mixed for 4 days with a Teflon magnetic stirrer (400 rpm). After stirring, the solution was left to stand overnight to allow the large undissolved SWCNT bundle aggregates to settle at the bottom. The dispersions were centrifuged in 15 mL glass vials at 4000 rpm for 1 h, and the top 2/3 of the black solution was collected and kept for later use. The concentration of SWCNTs in the processed solution was determined by dry extract. The concentration of both P2-SWCNTs and AP-SWCNTs in DMSO after processing was 0.1 mg/mL. Preparation of SWCNT Dispersions in Water (EdN). Inside the glovebox, a syringe was filled with nanotubide solution (the needle capped with a septum), taken outside the box and rapidly injected, under ambient atmosphere, into a glass vial containing a volume of previously degassed deionized water (Milli-Q water, 18.2 Mohm cm) G


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ACS Nano

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsnano.8b04341. XPS, Raman, and TGA characterization of AP-SWCNTs and P2-SWCNTs, peapod procedure and images, BALM images, calculation of the interaction energy between two individual SWCNTs. (PDF)

AUTHOR INFORMATION Corresponding Authors

*E-mail: [email protected]. *E-mail: [email protected]. ORCID

George Bepete: 0000-0002-5562-1125 Alain Pénicaud: 0000-0001-8614-2867 Carlos Drummond: 0000-0003-4834-3259 Present Address ∥

Department of Chemistry, The Pennsylvania State University, University Park, PA, 16802, United States Notes

The authors declare no competing financial interest.

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