Controlled Alignment of Individual Single-Wall Carbon Nanotubes at

Article pubs.acs.org/JPCC

Controlled Alignment of Individual Single-Wall Carbon Nanotubes at High Concentrations in Polymer Matrices Camilo Zamora-Ledezma,*,† Christophe Blanc,‡ and Eric Anglaret‡ †

Laboratorio de Física de la Materia Condensada, Centro de Física, Instituto Venezolano de Investigaciones Científicas, Altos de Pipe, 1204 Caracas, Venezuela ‡ Laboratoire Charles Coulomb, UMR CNRS 5521, Université Montpellier II, Place Eugène Bataillon, 34095 Montpellier Cedex 5, France ABSTRACT: We show that single-wall carbon nanotubes (SWCNT) can be dispersed as individuals in poly(vinyl alcohol) matrix composites up to concentrations of 1 wt %, as indicated by their strong photoluminescence (PL) signal in the near-infrared all along the processing steps. The alignment of the SWCNT is controlled by hot-stretching of the composite films. We show that orientational order can be described accurately from polarized Raman and PL spectroscopies, in good agreement with a simple affine model, providing their anisotropic absorption is properly taken into account. Similar results are obtained from different excitation laser lines (in the visible and near-infrared), confirming that all nanotubes orient the same way.

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functionalization.2 Covalent functionalization modifies the electronic properties, whereas physical adsorption of surfactants, polymers, or biomolecules is very efficient to disperse nanotubes in aqueous media and therefore suitable for further processing with only minor and reversible changes of the electronic properties. Homogeneous and stable CNT solutions can be also prepared in organic solvents such as N,Ndimethylformamide (DMF) or N-methylpyrolidone (NMP) without any surfactant but with limited amounts of nanotubes and incomplete exfoliation.13−15 The other crucial step is the (partial) alignment of the tubes through various techniques,16−18 either during the formation of the composite (using filtration, fiber drawing, magnetic and electric fields, hydrodynamics flow) or afterward (mechanical stretching or compression of polymers...). During the composite fabrication, the quality of dispersion can be controlled by microscopy methods on the micrometer scale (optical microscopy) or on the nanometer scale (scanning or transmission electronic microscopy). Optical spectroscopy signatures, especially absorbance, Raman, and PL, are also sensitive to intertube interactions on the nanometer scale, leading to some broadening of the spectral profiles, to some changes in the resonance conditions, and to a loss of the PL signal as soon as semiconducting nanotubes are connected to metallic ones.9−11 The same techniques can be used to measure anisotropy in macroscopic CNT based composites. Analysis of electronic microscopy images provides an estimation of nanotube orientation but is limited to small volumes of

INTRODUCTION The exceptional mechanical, thermal, electronic and optical properties of single-wall carbon nanotubes (SWCNTs) make them promising for a wide range of applications.1 In particular, they have been early identified as multifunctional nanofillers for preparing high-performance composites.2 However, the final properties of the composites will strongly depend on two issues: the homogeneous dispersion of the nanotubes in the matrices down to the individual scale (including exfoliation of nanobundles where intertube interactions affect most of their physical properties) as well as their orientation.3 This is particularly true for electrical and optical properties. For example, two key parameters to control conductivity in composites are the percolation threshold and the number of contacts between nanotubes. It is straightforward that both dispersion and orientation will allow us to decrease both these parameters4−8 and that this will significantly impact performances and cost. As far as optical properties are concerned, semiconducting SWCNTs are fluorescent in the near-infrared (NIR). Their photoluminescence (PL) is, however, quenched when they are assembled into bundles because of the contacts with metallic nanotubes, which open nonradiative relaxation channels for excited carriers.9 Furthermore, the PL signal is strongly polarized and much stronger for incident and emitted light polarized parallel to the nanotube axis. Therefore, PL of nanotube-based composites could be greatly increased provided the macroscopic orientation of the nanotubes is wellcontrolled.10,11 Various techniques have been used so far to disperse and align CNT in composites efficiently.12 A crucial step is often the preparation of stable suspensions of individualized carbon nanotubes, achieved by either covalent or noncovalent surface © XXXX American Chemical Society

Received: December 15, 2011 Revised: May 22, 2012

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dx.doi.org/10.1021/jp212113b | J. Phys. Chem. C XXXX, XXX, XXX−XXX

The Journal of Physical Chemistry C

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samples/small amounts of nanotubes. More statistical measurements can be obtained from X-ray diffraction19 or polarized light in optical techniques (birefringence measurements, polarized Raman, and PL).11,20 As far as SWCNTs are concerned, the most effective techniques are undoubtedly optical spectroscopy techniques. Each type of SWCNTs, defined by its diameter and chiral angle or equivalently by a pair of integers (n,m), presents specific absorption energies Eii corresponding to excitonic transitions associated with the ith symmetric pair of its van Hove singularities.9,11,21 It can thus be identified by its Raman signal from the couple (ΔνRBM, Eii), where ΔνRBM is the frequency of its radial breathing mode (RBM). The resonant Raman signal is strongly enhanced when the excitation laser line is close to Eii for excitation and scattered waves parallel to the nanotube axis. It can therefore be used to measure the orientational order parameter22 that quantitatively describes the tubes alignment. Finally, the PL of semiconducting SWCNT, at energy E11, is also resonantly enhanced when the excitation energy is close to Eii. A strong PL signal is, however, observed only when there are no contacts between semiconducting and metallic tubes; otherwise, nonradiative relaxation paths will quench the PL.9 Therefore, the observation of a strong PL signal can also be used as an indicator of a good dispersion of the SWCNT on the individual scale in aqueous suspensions and composites.10 The PL signal is also strongly polarized parallel to the nanotube axis and is also an indicator of the nanotube alignment.11 In a recent paper,11 we showed how combined Raman and PL spectroscopies could be used to measure accurately the order parameter of SWCNT dispersed as individuals in stretched PVA−matrices composites. This work was focused on the validation of the method, and only reference samples, that is, very diluted and transparent samples, were examined. In this Article, we show that SWCNT can be individually dispersed and aligned in PVA composites, at large concentrations, as high as 1 wt %. The observation of PL signals in each stage of the preparation and alignment indicates that the nanotubes remain individual or in small bundles. Several polarized Raman spectroscopies have been used to describe the alignment of the nanotubes. The same results are observed for different excitation lines, as well as for PL, showing that all types of nanotubes are similarly aligned. The alignment of individual nanotubes in the composites is obtained by stretching the composites above the PVA glass-transition temperature. We show that this alignment can be tuned from an isotropic distribution up to an order parameter of ∼0.8. Provided the anisotropic absorption of the samples is taken into account, the order parameter is found to be independent of the nanotube concentration and of the nature of the tubes, which is an additional confirmation of their good dispersion.

temperature. After this step, the mixture was rinsed out carefully to remove any trace of acid remaining in the sample. Hydrophilic Durapore membrane filters of HVLP of pore size 0.45 μm and abundant deionized water were used until a neutral pH was recovered. The final wet-cake was composed of nanotubes with iron content below 6 wt %. These nanotubes were initially assembled into interconnected bundles. They were dispersed and exfoliated under sonication in water using denaturated (i.e., shortened single stranded) DNA as dispersing agent for a final nanotube/DNA weight ratio (1:2). Stable and homogeneous aqueous suspensions were achieved, and the observation of intense PL signals in the NIR was used as an indication that most of the nanotubes are individual or in very small bundles in the suspensions.10,11 Macroscopic polymer films were obtained by mixing equal quantities of SWCNT suspensions with aqueous PVA solution (10 wt %) under moderate stirring during 15 min at room temperature. The mixture was transferred into Petri dishes and dried over 48 h at room temperature. Finally, self-standing films were peeled off from Petri dishes. Two series of samples were prepared with two different nanotube concentrations, 0.01 and 1 mg/mL, corresponding to final weight ratio of 0.01 and 1 wt % in dry composites. Figure 1 shows that such films are flat and optically homogeneous.

MATERIALS AND METHODS The procedure for the preparation of poly(vinyl alcohol) (PVA) films based on individualized SWCNTs was reported elsewhere.11 Raw nanotubes synthesized from carbon monoxide (HipCO) were purchased from Unidym. Raw material contains ∼25 wt % of iron particles used as catalyst during their synthesis. Therefore, raw nanotube powders were treated in a mild acid purification process23 with a preoxidation step during 12 h at 200 °C in air atmosphere. Oxidized nanotubes were then mixed with hydrochloric acid aqueous solution (37 wt %) with a nanotubes/HCl weight ratio (1:10000) and stirred vigorously with a magnetic stirring bar during 3 h at room

Alignment of the nanotubes in the composites was achieved by mechanical stretching (hot-drawing) at a velocity of 1.2 cm/ min inside an oven at 120 °C and then cooling to room temperature before releasing the stress. Because the amorphous PVA matrix is viscously deformed above its glass-transition temperature, Tg (85 °C), one can achieve high strains ε = Δl/l0 ranging from 0 to 4 (400%), which corresponds to elongation ratios λ = lf/l0 = ε + 1 ranging from 1 to 5, where l0 and lf are the initial and final lengths, respectively, and Δl = lf − l0. Raman measurements were carried out using three different excitation lines. In the visible, we used the green and red lines of an Ar/Kr laser (2.41 and 1.92 eV, i.e., 514.5 and 647.1 nm)

Figure 1. (a,b) Nonoriented self-standing DNA-wrapped SWCNT/ PVA with nanotube concentrations 0.01 and 1 wt %, respectively. (c,d) Typical sample before and after hot-drawing mechanical stretching. Scale bar, 15 mm.

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Figure 2. Raman spectra for isotropic diluted/concentrated composite films for excitation lines at: (a) 2.41, (b) 1.92, and (c) 1.17 eV. For comparison, PVA films spectra are shown for each excitation line. Peaks labeled with a star (*) are assigned to the PVA matrix. The spectra of the composites were slightly upshifted along the intensity axis for clarity.

double-resonance phenomenom implying elastic scattering of electrons by structural defects,24,25 and (iii) the strong so-called G band corresponding to CC stretching in graphitic materials, where the G− and G+ components, measured at 1560 and 1593 cm−1, respectively, are typical of SWNCT and assigned to transverse optic (TO) and longitudinal optic (LO) modes, respectively, for semiconducting nanotubes, and to LO and TO, respectively, for metallic nanotubes.24 Resonance occurs at different excitation lines for different values of the couple (n, m). At 2.41 eV, selective resonance occurs for metallic nanotubes with diameters in the range 0.8 to 1 nm and for semiconducting nanotubes with diameters in the range of 1.2 to 1.3 nm, as confirmed by the ranges of RBM frequencies. At 1.92 eV, selective resonance occurs for metallic nanotubes with diameters in the range of 1.2 to 1.3 nm and for semiconducting nanotubes with diameters in the range 0.8 to 1 nm, as confirmed by the ranges of RBM frequencies. Finally, at 1.17 eV, only small semiconducting nanotubes are in resonance, as illustrated by the RBM signature around 270 cm−1. Changes in the G band profile are also a consequence of these resonances: the G− profile is expected to be broader and asymmetric in the case of metallic nanotubes due to electron− phonon coupling,24 as notably observed on spectra excited at 2.41 eV and in a smaller extent for spectra excited at 1.92 eV. In general, the best choice to eliminate an extrinsic PL background in Raman measurements is to use low-energy excitation lines, typically in the NIR range. Another specificity of NIR excitation is the simultaneous observation of Raman and intrinsic NIR PL of the nanotubes. Indeed, no further extrinsic PL can be observed on PVA in Figure 2c. The broad bands superimposed to the Raman peaks of the composites are assigned to intrinsic PL from semiconducting SWCNTs. Their observation confirms that tubes remain well-dispersed in the composites and do not reaggregate during drying, even in the case of concentrated samples. Figure 3 displays typical polarized absorption and Raman spectra for an excitation line at 1.92 eV for two different nanotube concentrations and three different elongation ratios. Both absorption and Raman spectra are polarized, and the intensity ratio between spectra polarized parallel or perpendicular to the strain axis increases for increasing elongation ratio.

and a Jobin-Yvon T64000 spectrometer, operating in a single grating (1800 gr/mm) configuration. In the NIR, we used the 1.17 eV/1064 nm line of a Nd:YAG laser and a Fourier transform Bruker RFS100 spectrometer. The range of detection of the nitrogen-cooled Germanium detector associated with this spectrometer is 900−1700 nm. Therefore, NIR Raman signal and NIR PL from the nanotubes can be measured together in a single experiment. For all excitation lines, spectra were measured in three different polarization configurations: VV, VH, and HH, where the first and second subscripts refer to the polarizations of the incident and scattered beams, respectively, either parallel (V) or perpendicular (H) to the strain direction.

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RESULTS Figure 2 displays typical Raman spectra measured for three different excitation lines for diluted and concentrated isotropic samples. The Raman signal of SWCNT is strongly enhanced due to resonance and is therefore much stronger than the nonresonant signal of PVA (stars in Figure 2). Neither the nanotubes nor the PVA are expected to emit light in the visible. Therefore, the PL background observed with visible excitation lines is assigned to organic impurities or dangling bonds in DNA/matrix. This extrinsic PL is much stronger for an excitation line at 2.41 eV. This hinders the measurement of the SWCNT Raman signal for diluted samples. Therefore, the spectra measured at 2.41 eV will not be considered below for the calculation of the order parameter. By contrast, with an excitation line at 1.92 eV, one well observes all intrinsic Raman signatures of SWCNT; that is: (i) the radial breathing modes (RBMs), corresponding to radial in-phase motions of the carbon atoms, whose frequency is inversely proportional to the nanotube diameter and obeys the relation Δν (cm−1) = A/d (nm) + B, where A and B depend on the physicochemical environment of the nanotubes (individual or in bundles, air or water, surfactant or not...) (it has been reported from theoretical and experimental values that A can vary between 220 and 260 cm−1 and B can vary between 0 and 20 cm−1),24−28 (ii) the so-called D band around 1300 cm−1, corresponding to a zone boundary mode and therefore, in principle, inactive in Raman, which is activated through a C



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Figure 3. Polarized absorption spectra in the visible range (top) and Raman spectra for excitation line at 1.92 eV (bottom) for nanotube concentrations of 0.01 (left) and 1 wt % (right) and different elongation ratios λ. All Raman spectra were upshifted along the intensity axis for clarity.

system, was validated a posteriori by the perfect superposition of all polarized spectra by a simple multiplication factor.11 For a dilute system of SWCNTs, the backscattered polarized intensities measured for an incident intensity I0 are then expressed as a function of the orientation distribution of the tubes11 as follows

However, providing the extrinsic PL background is properly subtracted from the Raman spectra, Raman profiles are similar, that is, superimposable by a simple multiplication factor, in the VV and HH configurations. Note that Raman shift of the G and D* bands was reported in stretched composites and assigned to matrix-induced stress.29 In the present studies, no significant shift of these bands could be observed even for the strongest strains (dotted vertical lines in Figure 3). Indeed, because the composites are stretched above Tg, the stress acting on the tubes is rather weak due to the low yield stress of the PVA matrix.31 The orientational anisotropy of the nanotubes in the stretched composites is described by the order parameter S = /2, where β is the zenithal angle between nanotube axis and the strain direction, and the brackets indicate an average on all nanotubes. Assuming an uniaxial distribution and a single nonzero component of the Raman polarizability tensor,11,30 the order parameter can be obtained from Raman intensities measured in three different polarization configurations, that is, VV, HH, and VH. The second assumption, a priori valid for each vibrational mode in the case of a resonant

IVV = Cεzz 2I0⟨cos 4 β⟩ IHH = Cεzz 2I0⟨sin 4 β cos 4 θ ⟩ and IVH = Cεzz 2I0⟨cos2 β sin 2 β cos2 θ ⟩

where θ is the azimuthal angle, C is an experimental factor (including experimental constants such as the laser power, the scattering solid angle, and the detector efficiency), and εzz is the only nonzero component of the Raman polarizability tensor. For a concentrated system, however, incident and scattered light beams are absorbed differently along the V and H directions. Taking into account absorbances at the incident (Ei) and scattered (Es) energies and integrating on the film thickness d, the polarized scattered intensities express D



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Figure 4. (a,b) Polarized absorption spectra in the NIR range and (c,d) Raman spectra for excitation line at 1.17 eV for nanotube concentrations of (a−c) 0.01 wt % and (b−d) 1 wt %, respectively, and different elongation ratio λ. All Raman and PL spectra were upshifted along the intensity axis for clarity.

IVV = =

∫0

d

where the dichroism ratio is defined as Δ = A///A⊥. The expression is simplified in the case of a diluted (and/or thin) sample (A ≪ 1)

CεzzI010−(A//(λi) + A//(λf ))z / d⟨cos 4 β⟩ dz

Cdεzz I0(1 − 10−[A//(λi) + A//(λf )]) ln 10[A//(λ i) + A//(λf )]

S=

⟨cos 4 β⟩ IHH

⟨sin 4 β cos 4 θ ⟩

Cdεzz I0(1 − 10−[A//(λ i) + A⊥(λf )]) ln 10[A//(λ i) + A⊥(λf )] ⟨cos2 β sin 2 β cos2 θ ⟩

where A// and A⊥ are the film absorbances parallel and perpendicularly to the orientation axis, respectively. After integration over the azimuthal angle, the order parameter S can be computed from these expressions through S=

2⟨cos2 β⟩ + ⟨cos2 β sin 2 β⟩ − ⟨sin 4 β⟩ 2⟨cos 4 β⟩ + 4⟨cos2 β sin 2 β⟩ + 2⟨sin 4 β⟩

A simple expression is obtained in two limit cases. For a concentrated (and/or thick) sample (A ≫ 0), one gets S=

6ΔIVV + 3(1 + Δ)IVH − 8IHH 6ΔIVV + 12(1 + Δ)IVH + 16IHH

(2)

In Figure 4, we present typical polarized absorption and coupled Raman/PL spectra measured using an excitation line at 1.17 eV for two different nanotube concentrations and three different elongation ratios. All spectra are strongly polarized. Therefore, the same assumption can be made for Raman and PL; that is, only one component of the polarizability tensor relating the incident field and the scattered/emitted field is nonzero. Within this simple assumption, the order parameter of SWCNT dispersed as individuals in a composite can be measured accurately from polarized Raman and/or PL spectra.11 Figure 5 compares the order parameter, computed from the above expressions for diluted and concentrated samples, with the results (dotted lines) obtained from a simple analytical model assuming (i) an affine deformation of the polymer matrix, (ii) a reorientation of the nanotube axis imposed by the deformation of the matrix, and (iii) possible buckling for nanotubes initially oriented close to the perpendicular of the strain axis. (Details of this analytical model were presented in ref 11.) For both excitation lines, a good agreement is observed without any fitting parameters for diluted samples (open circles in Figure 5), as already reported.11 When absorption is neglected (eq 2), the apparent order parameter of the concentrated samples is much lower than the expected order

Cdεzz = I0(1 − 10−[A⊥(λ i) + A⊥(λf )]) ln 10[A⊥(λi) + A⊥(λf )]

IVH =

3IVV + 3IVH − 4IHH 3IVV + 12IVH + 8IHH

(1) E



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and PL. Finally, it is worth noting that in this study no residual stress could be measured on the nanotubes in the stretched composites because the composites were stretched above the glass-transition temperature of the polymer matrix.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the ANR P-NANO program and partially supported by a bilateral France-Venezuela PCP (Programa de Cooperación de Postgrados) and PICS ́ (Programa Internacional de Cooperación Cientifica).

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REFERENCES

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Figure 5. Comparison of the experimental orientational order parameter for (a) excitation line 1.92 eV and (b) excitation line 1.17 eV, with the corrected model for diluted and concentrated DNAwrapped SWCNT/PVA films. Dotted lines: analytical model; open circles: diluted samples; open triangles: concentrated samples without absorption correction (eq 2); and full triangles: concentrated samples with absorption correction (eq 1).

parameter (open triangles in Figure 5). However, when dichroism is taken into account (eq 1), a very good agreement is achieved without any fitting parameters for concentrated samples too (full triangles in Figure 5). Observing the same evolution for the orientational order during stretching for low and high concentrations is an additional confirmation of the good dispersion of nanotubes as individuals. Finding similar results from Raman and PL measurements, while PL is only sensitive to individual and Raman detects also the signal from bundles, also supports this conclusion. Finally, similar results are observed for two different laser lines, which show that the results are independent of the resonance conditions and therefore much likely valid for all kind of nanotubes.

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CONCLUSIONS SWCNTs can be dispersed as individuals in PVA−matrix composites up to concentrations as high as 1 wt %, as shown at each step of the processing by the PL signature. When the films are hot-drawn stretched, both Raman and PL signals get strongly polarized due to the tubes alignment. The orientational order has been investigated by several Raman excitation lines and PL spectroscopies, which give similar values (predicted by a simple geometric affine deformation model), suggesting that all nanotubes are oriented the same way. We showed that anisotropic absorption has to be taken into account for highly concentrated samples. The tubes remain individual in hot-drawn films, as supported by (i) the good agreement with the affine model, (ii) similar results for low and high concentration samples, and (iii) similar results for Raman F



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Controlled Alignment of Individual Single-Wall Carbon Nanotubes at

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