Hydroxide Ions Stabilize Open Carbon Nanotubes in Degassed Water

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Hydroxyl 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 ACS Nano, Just Accepted Manuscript • DOI: 10.1021/acsnano.8b04341 • Publication Date (Web): 08 Aug 2018 Downloaded from http://pubs.acs.org on August 9, 2018

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TOC Figure 128x62mm (144 x 144 DPI)

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Hydroxyl Ions Stabilize Open Carbon Nanotubes in Degassed Water George Bepete,1,2,# Nicolas Izard,3 Fernando Torres-Canas,1,2 Alain Derré,1,2 Arthur Sbardelotto,1,2 Eric Anglaret,3 Alain Pénicaud1,2* and Carlos Drummond1,2*

1

CNRS, Centre de Recherche Paul Pascal (CRPP), UMR 5031, F-33600 Pessac, France.

2

Univ. Bordeaux, CRPP, UMR 5031, F-33600 Pessac, France.

3

Univ. Montpellier, Laboratoire Charles Coulomb (L2C), UMR CNRS 5521, F-34000

Montpellier, France. # present address: Department of Chemistry, The Pennsylvania State University, University Park, PA, 16802, USA * [email protected] and [email protected] Keywords: carbon nanotubes – aqueous dispersions – photoluminescence – Raman – Eau de Nanotubes – hydrophobic interaction – nanotubide Abstract The main hurdle to the widespread use of single-walled carbon nanotubes remains the lack of methods to produce formulations of pristine, un-shortened, un-functionalized, individualized SWCNTs, thus preserving their extraordinary properties. In particular, sonication leads to shortening, detrimental to percolation properties (electrical, thermal, mechanical…). Using reductive dissolution and transfer into degassed water, open-ended, water filled nanotubes can be dispersed as individualized nanotubes in water/dimethylsulfoxide (DMSO) mixtures, avoiding the use of sonication and surfactant. Closed nanotubes on the other hand aggregate immediately upon contact with water. Photoluminescence and absorption spectroscopy both point out to very high degree of individualization, while retaining lengths of several microns. Resulting transparent conducting films are one order of magnitude more conductive than surfactant based blanks, at equal transmittance.

Physical 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 As1 ACS Paragon Plus Environment

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synthesized SWCNTs are intrinsically bundled due to the high attractive van der Waals forces between adjacent tubes (0.5 eV/nm).8 The SWCNT bundles consist of a mixture of semiconducting and metallic nanotubes which renders all the bundles metallic in nature.9–11 However, the most promising applications of SWCNTs in transparent conducting electrodes,12,13 biosensors,14,15 nanoelectronics,16 (opto)electronic systems, DNA sequencing, spintronics, 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 agents preventing reaggregation have been used, followed by ultra-centrifugation 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 on, 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, un-shortened, un-functionalized, individualized SWCNTs, thus preserving their extraordinary 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 aka 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/dimethylsulfoxide (DMSO) mixture, without surfactants nor sonication, provided 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 hence reduce the dielectric contrast with their environment. The stability 2 ACS Paragon Plus Environment

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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 (water filled) than for closed (vacuum filled) tubes. Consequently, vdW forces will be diminished (and the stability of the suspension augmented) for opened vs closed SWCNTs. RESULTS Closed (empty) vs opened (filled) tubes Carbon nanotubes are opened and water-filled by tip-sonication, a mostly unnoticed fact until the works of S. Cambré and W. Wenseleers38–40 but sonication had to be discarded since unshortened tubes are required. Closed and empty SWCNTs were obtained from Carbon Solutions, Inc.. They have been synthetized 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)). APSWCNTS (as produced) are closed-ended whereas P2-SWCNTs (purified) 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 micrometer) nanotubes at the end of our process (Figure S5)). The P2SWCNTs are obtained (by the supplier) by air oxidation and subsequent acid treatment purification of AP-SWCNTs. AP-SWCNTs and P2-SWCNTs both exhibit low and similar functionality (see XPS characterization in Supp. Info, 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 P2-SWCNTs 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 open-ended SWCNTs. We checked the opening, or not, of the tubes by preparing peopods42 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 inert atmosphere on a hot plate with SWCNT powder as has been reported for graphite intercalation.37,45,49 Stoichiometry was chosen as KC25 in order 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 3 ACS Paragon Plus Environment

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with stirring and no sonication, then centrifuging (4000 rpm for an hour) to remove un-dissolved tubes and catalytic material.52

Surfactant-free aqueous dispersion (Eau de nanotube):

Nanotubide solutions were then removed from the inert atmosphere glove box and rapidly injected into slightly basic water (pH 9). Three parallel experiments were performed: (i) opened tubes in normal (not degassed) water, (ii) opened tubes in degassed water, (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 & 1c). Experiment (ii) yielded a homogenous 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 in the following Eau de Nanotube (EdN). Carbon nanotube concentration in EdN was determined by dry extract as 26 mg.l-1 (after subtraction of the KOH fraction, with one KOH per 25 carbon atoms). Additionally, stability of EdN was tested against pH variation: 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 re-aggregation 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 of pH 9. It should be emphasized that AP-tubes contain more catalyst impurities than P2-tubes (ca 37% against 12%, see methods) 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.

Figure 1. (Meta) stable dispersions are prepared with opened tubes and degassed water. Pictures of the different dispersions of SWCNTs in water samples. (a) Re-aggregation 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.

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Evidences of Individualization of tubes A dispersion of open-ended carbon nanotubes with bile salt was prepared, tip-sonicated and ultracentrifuged in order to have a reference sample of individualized tubes. This additional sample was called BS-disp, for bile salt dispersion.38,53 Concentration in carbon nanotubes in BS-disp was determined by dry extract as 260 mg.l-1 (bile salt proportion was measured by TGA and subtracted). Absorption, Raman and PL 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 1400-1800 nm, 800-1200 nm 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 By 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 wellresolved 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 dielectric environments around the nanotubes are not very different. For instance, energy redshifts 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 Redshifts of several tens of meV (several tens of nm) 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


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

Photoluminescence. Figures 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. Three main peaks are observed for BS-disp at excitation/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 pionneer 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. On the other hand, the two peaks at low excitation energies are observed on EdN at (944 nm/1540 nm) and (954 nm/1588 nm) with a slight redshift in S22 excitation (and no shift in S11 emission), and the onset of a peak is also observed around (1000 nm/1580 nm). When normalized by the absorbance, as in Figures


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2b&c, PL intensity in EdN is ca half of that of BS-disp. This is noteworthy since bile salts are known to be some of the best surfactant to disperse SWCNT, maximizing the exciton quenching distances and therefore optimizing their PL signal60 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 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. Figures 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 (13001350 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 fig. 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, ie 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 types of nanotubes. Therefore, the use of several laser lines is particularly important in Raman studies of SWCNTs in order to get a better sampling over various diameters/chiral angles. For SWCNT diameters in the range 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 (fig. 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, respectively64 ii) at 785 nm, the profiles of the G band are significantly different with an intense and broad profile of the Gband (ie of the low-frequency component of the G bunch) for 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-NIR spectra. Accordingly, the RBM profiles are 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, as supported by the UV-vis-NIR spectra, 7 ACS Paragon Plus Environment

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leading to a more drastic selectivity of the excited nanotubes in Raman. On the other hand, differences in the RBM profiles might be due to a diameter-selectivity of the dissolution process,66 supported by the non-observation of the PL peak at 855/1540 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, 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/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/G intensity ratio is small for EdN (0.05, fig. 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/IG ratio than BS-disp (Fig 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 (Fig. 3a-b) and PET substrates. Films made by both techniques showed equivalent properties (Fig 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 one order of magnitude better conductivity than films made from BS-disp (Fig. 3c) as had been already seen for nanotubide films vs surfactant based films.72,73 The very close electrical performances obtained for as deposited films and annealed films made from EdN (Fig. 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 happen around 400 ºC.74 Since all films are rinsed with water, this would be less effective for thinner films where KOH is washed away more efficiently.


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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, as 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 superlong tubes dissolved in superacids, unshortened by sonication, lead us to believe that the EdN procedure has the potential to produce highly performing films from benign, degassed water suspensions.

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 is 4 µm. c) Transmittance vs sheet resistance comparisons of transparent conducting films (TCFs) made from EdN and from BS stabilized SWCNTs dispersions together with data from the literature. EdN1a: As made spray coated films; EdN2a : As made vacuum filtrated films; EdN1b : Annealed spray coated films; EdN2b : Annealed vacuum filtrated films. References: Parekh76; Wang177;Hecht75; Mirri71; Wang228

DISCUSSION Schäfer et al. have hydrogenated graphene using a Birch-type reaction where 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 single electron transfer from graphenide to water forming H radicals. Those radicals, in term, could then either react together to form molecular hydrogen or hydrogenate graphene. In their experiment, 2 equivalents of water per metal atom, were added to liquid NH3 at -78 °C and got immediately frozen and only slowly released when the temperature is . increased, favoring low local concentrations of H radicals, 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. Since DMSO and water mixing is an exothermic process,78 temperature rose up 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 (Fig. 3c), contrary to graphene films from graphene water35 where defects are removed by annealing. Hence, a chemical sequence in agreement with the experimental observations is: 9 ACS Paragon Plus Environment

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.

.

C25 - K+ + H2O -> C250 + 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 to image 4 and 8 micrometer 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 forming 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 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 when the negatively charged open-ended nanotubes in DMSO meet degassed water, ensuring that individualized nanotubes are instantaneously mixed with degassed water causing hydroxide anion adsorption (as typically happens when a hydrophobic surface is in contact with water83,84) and electrostatic intertube repulsion. 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 by OHadsorption) 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


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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 zeta potential ζ (- 67 mV) from the measured electrophoretic mobility values (a description of the data treatment is presented in the SI). 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 with 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 supporting information while the result, a rough estimate of the SWCNT-SWCNT interaction, calculated by adding together 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 1 µm, diameter 1.5 nm) an increment in the energy barrier of the order of 17 kBT is obtained after filling the SWCNT with water. As 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


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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 which considers the nature of the SWCNTs and the real properties of confined water is required for an accurate description of the system under study.

Figure 4. a) Probability distribution of measured electrophoretic mobility of the SWCNTs in EdN. b) Interaction between parallel graphene-like cylinders vs surface-surface distance D calculated as described in the text and supporting information. Salt concentration 5mM. Cylinder length 1 µm; diameter 1.5 nm. T 298 K.

Conclusion 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 nor coated. EdN opens perspectives for organic, printable electronics, sensors and other devices. TCFs from EdN have been demonstrated. Potent SWCNT sorting capabilities by electronic character and indices on (shortened) surfactant based aqueous dispersions have been demonstrated.24,93 Preliminary experiments show that EdN can be prepared with surfactant containing water (actually 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 have a shelf life of a few weeks / months under refrigeration at 3–6 oC.


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Methods Materials. Studies were carried out using as-prepared single walled carbon nanotubes (AP-SWCNTs) and purified single walled carbon nanotubes (P2-SWCNTs) prepared by the electric arc discharge method using a Nickel/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. AP-SWCNTs 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), whilst 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 whilst AP-SWCNTs are capped. The P2-SWCNTs were completely transformed to peapods (filled with fullerene molecules), whilst 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 glove box 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 oC whilst mixing continuously for 10 minutes 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 minute interval for 1 hour, then every 30 minutes for 4 hours. The product was allowed to cool down to room temperature and the vial was tightly capped for later use. The salts were labeled AP-SWCNTs_KC25 and P2-SWCNTs_KC25 for AP-SWCNTs and P2-SWCNTs respectively. Inside the glove box, 4 mg of P2-SWCNTs_KC25 salt (respectively AP-SWCNTs_KC25 salt) was dispersed in 20 mL of dry analytical grade DMSO solvent (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 four days with a teflon magnetic stirrer (400 revolutions per minute (rpm)). After stirring, the solution was left to stand overnight to allow the large un-dissolved SWCNT bundle aggregates to settle at the bottom. The dispersions were centrifuged in 15 ml glass vials at 4,000 rpm for 1 hour and the top two thirds 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 P2SWCNTs and AP-SWCNTs in DMSO after processing was 0.1 mg/mL.


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Preparation of SWCNT dispersions in water (EdN). Inside the glove box, 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 (MilliQ water, 18.2 Mohm cm) 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 3-6 oC 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. Then the air pressure was gently increased back to atmospheric pressure. Bile salt dispersions BS-disp for comparison. 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 freshly deionized water. The mixture was tip-sonicated for 30 mins 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 SWCNTs, 126 mg Bile salt and 21 g of water. This diluted dispersion was ultracentrifuged at 150 000 g for 30 minutes in a Sorvall WX Ultra Series centrifuge from Thermo-Scientific. After separation of the centrifugate, the supernatant was centrifuged again at 150 000 g for 90 minutes. TGA measurements of the dry extract showed a content of 5 % SWCNTs and 95 % bile salt. Absorption spectroscopy. Absorption spectra were recorded in 10mm optical path quartz

cuvettes using a UV-VIS-NIR spectrometer (JascoV-670), in the wavelength range of 3801350nm. A baseline correction was performed using a DMSO/water mixture for EdN and BS solution for BS-disp. Photoluminescence excitation (PLE) experiments. Photoluminescence spectroscopy was performed using an home-built µ-PL setup. The output of a titanium-sapphire laser (excitation wavelength ranging from 800 nm to 990 nm) was focused onto the sample (inside a quartz-cell, near the entrance quartz windows) with an infrared microscope objective (40x, 0.45 N.A.). PL was collected by the same objective (in backscattering geometry), and direct laser line was rejected by a longpass 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. Excitating 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).


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Raman spectroscopy. Raman spectroscopy was performed on an Xplora spectrometer from HoribaJobin-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 (TCFs) 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/conductivity ratio (cf Fig. 3c). Sheet resistance measurements were done using a fourpoint probe station. Films by spray: Films were made using an ultrasonic spray robot. The glass substrates were fixed onto a hot plate at 90 ºC during spraying to evaporate the solvent immediately after coating. The films were rinsed by washing in successive water baths to remove KOH/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/Bile salts off 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 minutes in each bath. Acknowledgements We thank S. Cambré and W. Wenseleers for fruitful discussions and advices, M. Monthioux and L. Noé for the preparation of peapods and the corresponding TEM image S3, D. Ausserré and Watch Live S.A.S. (Lyon, France) for BALM image S4, C. Labruguère and Placamat for the XPS analysis. Support from the Linde Corporation is acknowledged. This work was carried out within the framework of GDR-I Graphene and Co. Supporting information available: XPS, Raman and TGA characterization of AP-SWCNTs and P2SWCNTs, peapod procedure and images, BALM pictures, calculation of the interaction energy between two individual SWCNTs. This material is available free of charge via the Internet at http://pubs.acs.org. References (1)

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