Highly Efficient and Predictable Noncovalent ... - ACS Publications

Article pubs.acs.org/JPCC

Highly Efficient and Predictable Noncovalent Dispersion of SingleWalled and Multi-Walled Carbon Nanotubes by Cellulose Nanocrystals Jean-Bruno Mougel,†,‡ Coline Adda,† Patricia Bertoncini,† Isabelle Capron,‡ Bernard Cathala,‡ and Olivier Chauvet*,† †

Institut des Matériaux Jean Rouxel (IMN), UMR CNRS 6502, Université de Nantes, 2 Rue de la Houssinière, BP 32229, 44322 Nantes cedex 3, France ‡ Institut National de la Recherche Agronomique (INRA), UR 1268, Rue de la Géraudière, BP 71627, 44316 Nantes cedex 3, France S Supporting Information *

ABSTRACT: Cellulose nanocrystals (CNCs) are shown to be able to disperse in a very efficient way both single-walled (SWNTs) and multiwalled carbon nanotubes (MWNTs). Optimization of the processing parameters (sonication time and power) leads to dispersion yields as high as 70 wt % for both types of carbon nanotubes (CNTs). Such a high dispersion yield obtained in a noncovalent way with biobased nanoparticles is noteworthy and deserves further attention. Atomic force microscopy and transmission electron microscopy images suggest that the CNCs and the nanotubes form hybrids, with the stabilization of the dispersion arising from both the irreversible adsorption of the CNCs onto the nanotubes and the electrostatic repulsion between the CNCs. A quantitative model is proposed, revealing that one CNC can stabilize one SWNT three times its length in the aqueous dispersion and that more CNCs are required in the case of MWNTs. This model allows us to control the dispersion yield as a function of the processing parameters. Cellulose is a linear homopolymer composed of β(1,4)-Dglucose residues that are self-assembled into fibrils with alternative crystalline and amorphous parts. The crystalline parts, known as cellulose nanocrystals (CNCs), can be extracted by sulfuric acid hydrolysis,25 producing rod-like nanoparticles. CNCs are highly organized crystals with dimensions that depend on the origin of the cellulose.26 In this study, CNCs are obtained from cotton and have a rod-like morphology with an average cross-section of 5 to 13 nm and a length of ∼200 nm. These CNCs exhibit interesting mechanical characteristics and faceted surface chemistry.27 The acid hydrolysis introduces some ester sulfate groups at the surface of the crystals, leading to stable colloidal suspensions as a result of electrostatic repulsion. Moreover, CNCs have four different identified crystalline planes that vary in chemical affinity. As a result, they are mainly hydrophilic materials that exhibit amphiphilic characteristics, with one plane that is more hydrophobic (200).28 The ability of CNCs to stabilize oil/ water interfaces illustrates that behavior.29 In a previous paper,30 we have shown that CNCs can very efficiently disperse single-walled carbon nanotubes (SWNTs) in water to form colloidal dispersions that remain stable for months. These dispersions were successfully used to grow

1. INTRODUCTION Since their discovery by Iijima,1,2 carbon nanotubes (CNTs) and especially single-walled nanotubes (SWNTs) have been the focus of great interest due to their unique optical, mechanical, and electronic properties,3,4 which are strongly dependent on their structures and environment.5 These properties give them the ability to be used for many applications in various areas such as material science,6 electronics,7 and biology.8 However, the poor solubility of CNTs in water, partly due to the hydrophobicity of the carbon surface and the highly attractive van der Waals forces9 between them (leading to bundling), makes the process difficult to implement. Among the different strategies that can be used to overcome this poor solubility, noncovalent functionalization10 appears to be very promising because it makes it possible to maintain the optoelectronic properties of SWNTs. It has been successfully applied using surfactant molecules11−15 and polymers (DNA,16 block copolymers,17 PMMA,18 PPV19). For biomedical applications,20 it is suitable to make use of biobased dispersants. For example, hydrosoluble polysaccharides such as gum arabic,21 chitosan,22 or cellulose derivatives23,24 have recently been identified as efficient dispersants of SWNTs, opening the way to the dispersion of SWNTs in biocompatible media. These biomolecules might desorb from the surface and hinder the access to CNT limiting their use in numerous applications. A new opportunity comes with associated biobased hybrid nanoparticles. © XXXX American Chemical Society

Received: July 20, 2016 Revised: September 9, 2016

A

DOI: 10.1021/acs.jpcc.6b07289 J. Phys. Chem. C XXXX, XXX, XXX−XXX

Article

The Journal of Physical Chemistry C

Figure 1. (a) TEM micrograph of dried CNCs. (b) CNC suspensions at 17 (left) and 4 g/L (right). (c) SWNT/CNC and (d) MWNT/CNC dispersions in water.

multilayered thin films with interesting optical properties. In this work, we show for the first time that multiwalled carbon nanotubes (MWNTs) can be efficiently dispersed in water by CNCs as well with similar yield. A parametric study of the dispersion process for both SWNTs and MWNTs allows understanding and controlling the dispersion mechanism for each system. The effects of the sonication conditions and the initial concentrations of CNTs and CNCs on the yield of the dispersion process are investigated, whereas the final quantity of CNTs is determined by optical absorption measurements. The morphology of the SWNT/CNC and MWNT/CNC hybrids is explored using atomic force (AFM) and transmission electron microscopies (TEM). Combining all of these results allows us to propose a quantitative understanding of the association mechanisms between CNTs and CNCs. Long-term dispersion with yields higher than previously reported can be reached in a predictable way.

experiment. Approximately 2 mg of SWNTs (or 3 mg of MWNTs) was added to 3 mL of CNC suspension, followed by cup-horn sonication (Bioblock Scientific, Vibra-cell 75115 operating at 20 kHz) with a power of 0.7 W/mL for a sonication time of 2.5 h (or 1 h for MWNTs). A cooling bath was used at 4 °C. After a centrifugation step (20 000g, 30 min), the remaining CNT bundles were precipitated and the supernatant was collected. TGA measurements revealed that the CNC concentration remained constant after the centrifugation step. A parametric study of the dispersion process was conducted versus the sonication time tUS (min), the power density PUS (W/mL), and the initial weight concentrations of the CNCs ([CNC]), SWNTs ([SWNT]), and MWNTs ([MWNT]). 2.2.3. Absorption Spectroscopy. The final concentration of CNTs in the dispersion was obtained through optical absorption using a PerkinElmer Lambda 1050 UV−vis/NIR spectrometer. The concentration was calculated according to Haggenmueller et al.13 The extinction coefficients were measured at 891 nm and at 951 nm for the SWNT/CNC and MWNT/CNC hybrids, respectively (see the Supporting Information for details). The yield of the dispersions ηSWNT is defined as ηSWNT = [SWNT]final/[SWNT]initial. 2.2.4. Transmission Electron Microscopy. The samples were prepared by depositing 20 μL of a 10-fold diluted dispersion onto a glow discharged carbon-coated electron microscope grid (200 mesh, Delta Microscopies, France) for 2 min. Excess solution was removed by blotting. The sample was then negatively stained with phosphotungstic acid solution (1% w/v) and dried just before observation. Images were recorded using a JEOL 1230 TEM operating at 80 kV. 2.2.5. Atomic Force Microscopy. Positively charged poly-Llysine (20 μL) was deposited as an anchoring layer on a freshly cleaved mica substrate, rinsed with water, and dried under a nitrogen flow. The same volume of the dispersion containing CNT/CNC hybrids was then deposited, rinsed and dried. AFM images were recorded in air using the PeakForce Quantitative Nanomechanical Properties Mapping mode of the Multimode 8 Nanoscope V (Bruker). Silicon nitride tips were used (ScanAsyst Air, Bruker). Heights on AFM images were measured using WSXM software routines.31

2. EXPERIMENTAL SECTION 2.1. Materials. Commercially available HipCO SWNTs were purchased from Unidym (batch P 062) (Sunnyvale, CA) and used as received. Their diameters ranged from 0.8 to 1.2 nm and their lengths from 100 to 1000 nm (according to the manufacturer’s specifications). MWNTs were purchased from Nanocyl (NC 7000) (Sambreville, Belgium). Their average diameter was 9.5 nm and their average length was 1.5 μm (according to the manufacturer’s specifications). CNCs were prepared from Whatman filter paper (grade 20 CHR, VWR). 2.2. Methods. 2.2.1. Preparation of Cellulose Nanocrystal Suspensions. CNCs were extracted from cotton linters using a technique adapted from Revol et al.25 In brief, 7 g of cotton linters were added to 250 mL of distilled water and mechanically stirred for 24 h. Sulfuric acid (H2SO4) was added to obtain a final acid concentration of 58% and heated for 20 min at 70 °C. The acid hydrolysis was then stopped by diluting the mixture 5-fold with distilled water. The suspension was then centrifuged (10 000 rpm, 10 min), dialyzed to neutrality against Milli-Q water, and deionized using mixed bed resin (TMD-8). The final dispersion was sonicated for 20 min (Q700 sonicator; QSonica, Newtown, CT), successively filtered through 5 and 1.2 μm filter units (Whatman), and subsequently stored at 4 °C. A surface charge density of 0.1 e/nm2 of half ester sulfate groups at the surface of the CNCs was determined by conductometric titration with a NaOH solution using a conductivity module (Metrohm). The dimensions of our CNCs were investigated with transmission electron microscopy (Figure 1a). An average length and width of 120 and 10 nm were found, respectively. 2.2.2. Dispersion Preparation. CNC suspension was adjusted to 3.8 g/L in pure water and sonicated before each

3. RESULTS CNCs are nanorods that disperse in water with colloidal stability due to negative sulfate groups that promote electrostatic repulsion,32 as shown in Figure 1b. In a previous paper,30 we showed that CNCs have the ability to disperse SWNTs after ultrasonic treatment in the absence of any surfactant. Here we evaluate their ability to disperse MWNTs as well, and we B


Article


Figure 2. Evolution of

[SWNT]final [SWNT]initial

with the different processing parameters of the dispersion. Evolution with the sonication power PUS (a), the

sonication time tUS (b), the CNC concentration [CNC] (c), and the SWNT concentration [SWNT] (d). The fixed parameters are given in each panel. The red lines of panels c and d are obtained with the model described in the Discussion (eq 3).

were also obtained in the case of carboxymethylcellulose and chitosan, respectively.13 As can be seen in Figure 2a, a yield of 50% is obtained, irrespective of the sonication power density (in our range of power), for SWNT/CNC dispersions, but decreases from 65 to 30% in the case of MWNT/CNC dispersions (Figure 3a). Better yields are obtained when increasing the sonication time (Figures 2b and 3b) to a plateau value for both types of CNTs. Saturation appears earlier for MWNTs than for SWNTs. In general, the yield also increases with the CNC concentration (Figures 2c and 3c) and decreases in a nonlinear way with the CNT concentration (Figures 2d and 3d). This study allows us to obtain the optimal parameters of the dispersion. In SWNT/ CNC and MWNT/CNC dispersions, yields as high as 72 % and 80% are obtained with a power density of 0.7 W/mL and with a sonication time of 1 (for MWNTs) and of 2.5 h (for SWNTs), respectively. These processing conditions were used for the experiments illustrated in Figures 2c,d and 3c,d. To understand the dispersion mechanism, the morphology of the hybrids is investigated using microscopy techniques. Representative TEM and AFM images of SWNT/CNC and MWNT/CNC hybrids are shown in Figures 4a,b and 5a,b, respectively. Bare and covered areas can be seen on the CNTs on both types of hybrids. Elongated structures of several hundred nanometers can be attributed to CNTs, and those with a length of ∼120 nm can be attributed to CNCs (see Figure 1a for a comparison). This is confirmed by the height profiles recorded across the CNTs, as shown in Figure 4c for SWNT/CNC hybrids and in Figure 5c for MWNT/CNC. In fact, the height profiles shown in Figure 4c exhibit heights of

investigate the processing parameters to optimize the dispersion. Figure 1c,d shows photographs of stable dispersions of SWNT/CNC and MWNT/CNC hybrids, respectively. It can be seen that black, homogeneous dispersions are obtained. These dispersions are highly stable because they resist centrifugation for 30 min at 20 000g without visible sedimentation and may remain unchanged over months. To optimize the dispersion yield, we performed a parametric study. The yield, that is, the efficiency to maintain CNTs in suspension, is determined by measuring the absorbance of suspensions after centrifugation using UV−vis spectroscopy. The evolution of the yield is investigated with the different processing parameters by varying the sonication time tUS (from 15 to 210 min), the sonication power PUS (from 0.7 to 1.3 W/ mL), the initial concentration of the CNCs (from 1 to 10 g/L), and the initial concentration of SWNTs (from 0.4 o 1.7 g/L) or of MWNTs (from 0.2 to 1.3 g/L). The results are presented in Figure 2a−d for the SWNTs and in Figure 3a−d for the MWNTs. In both cases, we succeeded in dispersing CNTs with a yield as high as 72% for SWNTs (Figure 2c) and 80% for MWNTs (Figure 3c). These results can be compared with those obtained with conventional surfactants like SDS (sodium dodecyl sulfate) and SDBS (sodium dodecylbenzenesulfonate).12,13 An efficiency of 50% is reported by Haggenmueller et al.13 with SDBS and SDS under conditions similar to ours (sonication time: 2 h, 1.5 W/mL; centrifugation: 21 000g for 2 h; and initial SWNT concentration: 0.5 g/L with a ratio of SWNTs/surfactant by mass of 1:10). Yields of 50 and 40% C


Article


Figure 3. Evolution of

[MWNT]final [MWNT]initial

with the different processing parameters of the dispersion. Evolution with the sonication power PUS (a), the

sonication time tUS (b), the CNC concentration [CNC] (c), and the MWNT concentration [MWNT] (d). The fixed parameters are given in each figure. The red lines of panels c and d are obtained with the model described in the Discussion (eq 4).

Figure 4. (a) TEM micrograph of SWNT/CNC hybrids. (b) Topographical AFM image of SWNT/CNC hybrids and CNCs. (c) Height profiles recorded along the blue and red lines on the image. The hybrids shown here were obtained by preparing a dispersion that was sonicated for 2.5 h at 0.7 W/mL. The final CNC and SWNT concentrations are 3.8 and 0.27 g/L, respectively.

Figure 5. (a) TEM micrograph of MWNT/CNC hybrids. (b) Topographical AFM image of MWNT/CNC hybrid and CNCs. (c) Height profiles recorded along the blue and red lines on the image. The hybrids shown here were obtained by preparing a dispersion that was sonicated for 2.5 h at 0.64 W/mL. The final CNC and MWNT concentrations are 3.8 and 0.35 g/L, respectively.

D


Article

The Journal of Physical Chemistry C approximately 1 to 2 nm, corresponding to the diameter of a single SWNT or a small bundle of them and heights of approximately 8−16 nm, corresponding to the staking of CNCs (6 to 13 nm thick) and SWNTs. There are significant differences when looking at MWNT/CNC hybrids. In Figure 5c, the height measured across a bare part of a MWNT is ∼9 nm, corresponding to its diameter, and approximately 15 to 16 nm when the MWNT is covered by a CNC. Both TEM and AFM observations show CNCs aligned along the SWNTs, confirming our previous results.30 We attribute the alignment of the CNC along the SWNT to hydrophobic interactions that take place between the (200) surface crystalline plane of the amphiphilic CNCs28 and the sidewall of the SWNTs. In the case of MWNT/CNC hybrids, TEM and AFM observations show that CNCs are not necessarily perfectly aligned along the MWNT sidewall. The MWNTs used in this work have a mean diameter of 10 nm, so their surface curvature is much weaker than those of SWNTs. Therefore, the hydrophobic interactions between MWNTs and CNCs do not require a perfect alignment of both objects. Moreover, the MWNTs have many defects and are not straight over long distances (micrographs not shown here). These defects are probably associated with oxidation, which leads to the presence of hydrophilic groups on their sidewall, decreasing the effective hydrophobic interaction strength.

Table 2. Correlation Coefficients from the Parametric Analysis for the MWNTs

[CNC] [MWNT]init sonication duration ultrasound power yield

Table 1. Correlation Coefficients from the Parametric Analysis for the SWNTs

[CNC] [SWNT]init sonication duration ultrasound power yield

sonication duration

ultrasonic power density

1 −0.09 0.07

1 0.30

1

−0.07

−0.13

0.15

1

0.63

−0.37

0.52

0.11

[MWNT]init

sonication duration

ultrasonic power density

1 −0.05 −0.03

1 −0.11

1

−0.04

−0.13

−0.05

1

0.52

−0.54

0.38

−0.32

yield

1

power is no longer an important parameter. This is exactly what we observed: The yield is practically independent of the power (the correlation coefficient is low: 0.11). Conversely, several attempts were made to disperse SWNTs using a sonication bath, that is, at a very small power (PUS = 0.035 W/mL) while keeping the other parameters constant. Dispersions were still obtained but with a much lower yield of ∼1%, suggesting that there is a threshold to overcome in the case of SWNTs and that PUS = 0.035 W/mL is below the threshold (it is insufficient to initiate a suitable debundling). In the case of MWNTs, the situation is more complex. Indeed, the power needed for disentangling must be weaker than the one for debundling because MWNTs form networks and are only subjected to binding at the contact junctions, as discussed by Huang et al.33 This is confirmed by an attempt to disperse MWNTs with CNCs using a sonication bath (PUS = 0.035 W/mL). An efficient dispersion is achieved with a yield of ∼20%, whereas under the same conditions, dispersion of SWNTs is almost impossible. The yield tends to decrease when increasing the power, as demonstrated by the correlation coefficient of −0.32, seen in Table 2. Two explanations can be proposed: The ultrasonic energy is high enough to dissociate MWNT/CNC hybrids or ultrasounds induce defects on the outer wall of the MWNTs that are not favorable for the association of the two objects. Note that a shortening of the MWNTs with the length divided by 2 (TEM data not shown) was observed for power ranging from 0.7 to 1.1 W/mL. A shortening induced by ultrasonication was also reported for SWNTs11 and MWNTs.34 While an increase in the sonication power leads to a more violent implosion of the cavitation bubbles,35 increasing the sonication duration generates more bubbles in the system. We assume here that the dispersion process follows the mechanism of debundling proposed by Strano et al.36 The implosion of a cavitation bubble creates enough shearing to isolate a part of a CNT that can be associated with CNCs. The implosion of another bubble makes it possible to isolate a longer part of the nanotube, which may interact with another CNC and so on. Indeed, such a process may explain why the yield increases with the sonication time. It seems to be faster with MWNTs than with SWNTs. The fact that MWNTs are entangled while SWNTs are bundled may explain this difference. A higher yield value of 70% is reached after a sonication time of 100 min. Increasing the sonication time once again does not increase the yield. This suggests that the maximum disentanglement is achieved within these processing conditions. In the following, we will discuss how this maximum is related to the CNC and CNT concentrations. 4.2. Evolution of the Yield with CNC and CNT Concentrations. As can be seen in Figures 2c and 3c, it seems that for a given sonication power, sonication time, and

4. DISCUSSION CNCs can thus be efficiently used to disperse SWNTs and MWNTs. This result is interesting by itself because this is the first example to our knowledge of a high-yield dispersion obtained with biobased rigid nano-objects. The amount of CNTs dispersed in the aqueous solution depends on the sonication time and on the initial concentrations of CNCs and CNTs. The yield is only weakly dependent on the ultrasonic power density in the case of SWNTs, while it is not true in the case of MWNTs. These interdependencies are clearly highlighted when computing the correlation coefficient between all of the parameters, as illustrated in Tables 1 and 2.

[CNC] [SWNT]init

[CNC]

yield

1

4.1. Role of Ultrasounds. Physically, SWNTs and MWNTs have different dimensions and are associated in different ways. As produced SWNTs appear to be aligned in densely packed bundles, whereas MWNTs are organized in an entangled network. Therefore, two conditions must be satisfied to disperse CNTs and to stabilize the dispersions: (i) first, the debundling of SWNTs or the disentangling of MWNTs and (ii) second, the occurrence of an interaction between the nanotube and the CNCs. The role of ultrasounds through the cavitation process is to initiate the debundling or the disentangling. In the case of SWNTs, once debundling is initiated, the sonication E


Article

The Journal of Physical Chemistry C Table 3. Relationship between the Length of SWNTs and the Number of CNCs Hanging onto the Tubes number of CNCs associated per SWNT

1

2

3

4 or more

mean length of SWNTs (nm)

336 (±172)

647 (±242)

893 (±280)

1208 (±514)

concentration of CNTs, the final concentration of CNTs in suspension, and thus the yield, increases linearly with the concentration of CNCs. Linear fits for [CNC] < 5 g/L are as follows ηSWNT =

0.055 × [CNC] [SWNT]init

ηMWNT =

0.053 × [CNC] [MWNT]init

by hydrophobic interactions between the (200) crystalline plane of the CNCs and the SWNT sidewall. Because of the small diameter of the nanotubes compared with the crosssection of the CNCs and the strong distance dependence on the hydrophobic interaction, such a mechanism induces the alignment of the two nano-objects. With such a morphology, the dispersion stability of the hybrids, like for CNCs alone, is due to electrostatic repulsion between the CNCs. At this point, a relationship may exist between eq 3 and the morphology of the hybrids. The mass of a SWNT with diameter d and length l is obtained by multiplying the surface density of the graphene sheet37,38(7.6 × 10−7 kg·m−2) by the surface of the SWNT without taking into account the extremities. It is given by mSWNT = π × d × l × 7.6 × 10−7 (kg). Assuming that the CNCs have a parallelepiped shape, the mass of a CNC with length l, height h = 10 nm, and width w = 10 nm is mCNC = w × h × l × 1640 (kg) (the density is 1.64 g·cm−3). The ratio of both masses is thus 0.015 per unit length. It is noteworthy that this ratio is on the same order of magnitude as the coefficient of 0.055 found in eq 3, suggesting that the stoichiometry of the hybrids is related to the ability of the CNCs to associate along the SWNTs. If we use the value of 0.055 derived from eq 3, we find that it corresponds to a length ratio lswnt/lCNC of 3.67, implying that one CNC can stabilize one SWNT three to four times its length. AFM observations support this point. In fact, by measuring the length of the SWNTs and counting the numbers of CNCs adsorbed onto them, we find that there is one CNC per SWNT length of roughly 310 nm (see Table 3). We recall here that the characteristic length of a CNC is 120 nm. In the case of MWNTs, the fact that the diameter is 10 times greater implies that the degree of surface bending is less than for SWNTs and that the hydrophobic interaction does not require perfect alignment between the nano-objects. Using the same approach as above, a ratio of mMWNT/mCNC of 0.981 per unit length is obtained. The MWNT mass is obtained by adding the mass of each wall with diameter di and length l. Therefore, the mass of ten layer MWNT is given by mMWNT ≈ −7 π × ∑10 (kg). The experimental coefficient i=1 di × l × 7.6 × 10 of 0.053 given by eq 4 is ∼20 times lower, confirming that the alignment of both objects is not the key parameter controlling the stoichiometry and that a much smaller number of MWNTs can be maintained in the dispersion by the CNCs than in the case of SWNTs. This is in agreement with our AFM and TEM observations, where it can be difficult to find MWNTs among the CNCs, perhaps due to the fact that the interaction between CNCs and MWNTs is much weaker than with SWNTs, presumably due to the large number of defects on the nanotube surface. As a consequence, more CNCs are needed to stabilize one MWNT than one SWNT of equal length.

(1)

(2)

The final concentrations of dispersed CNTs are thus related to the CNC concentration as follows [SWNT]final = 0.055 × [CNC]

(3)

[MWNT]final = 0.053 × [CNC]

(4)

The results of eqs 3 and 4 are illustrated as red lines in Figures 2c and 3c, respectively. These equations should remain valid as long as there are enough CNTs to be dispersed. Conversely, saturation is expected when all of the CNTs are dispersed, that is, for a yield of 100%. Equations 3 and 4 show that a yield of 100% might take place for CNC concentrations of 10 g/L for SWNT/CNC hybrids and 6.3 g/L for MWNT/ CNC hybrids. This saturation behavior is illustrated in Figures 2c and 3c. As can be seen, this very simple model quite accurately describes the evolution of the yield with the concentration of CNCs or of CNTs. Nevertheless, yields of 100% are not obtained experimentally. We assume that this may be due to remaining iron catalysts and other carbon structures that cannot be dispersed. Equations 3 and 4 thus suggest that the final composition of the dispersion is limited by stoichiometry. It should also determine the yield dependence in the case of CNTs. In Figures 2d and 3d, we plotted the results of eqs 1 and 2 with [CNC] = 3.8 g/L. As we can see, the model without any further parameter very well describes the experimental data for both types of CNTs. It shows that the decrease in the yield at high CNT concentrations is due to the limited amount of CNCs in the dispersion that cannot disperse more CNTs. The saturation observed in Figures 2b and 3b is also related to the required stoichiometry between CNTs and CNCs in the dispersion state. The expected values according to eqs 1 and 2 are ηSWNT =

0.055 × [CNC] 0.055 × 4 = = 0.37 [SWNT]init 0.6

and ηMWNT =

0.053 × [CNC] 0.053 × 4 = = 0.64 [MWNT]init 0.33

in good agreement with the observations. It is noteworthy that despite the very different aspect ratios of SWNTs and MWNTs the same stoichiometric coefficient of 0.055 ± 0.002 for both MWNTs and SWNTs is found. 4.3. Morphology of the CNT/CNC Hybrids. As already discussed, Figure 4a,b suggests that the CNCs are aligned along the SWNT axis, and we assume that this association is governed

5. CONCLUSIONS We investigate the ability of CNCs to disperse SWNTs and MWNTs in water. The dispersions are highly stable, showing that irreversible adsorption and high yields are obtained with >70 wt % of nanotubes dispersed (either SWNTs or MWNTs). A parametric analysis shows that the sonication time plays a key F


Article


(7) Avouris, P.; Chen, Z.; Perebeinos, V. Carbon-based electronics. Nat. Nanotechnol. 2007, 2, 605−615. (8) Lu, F.; Gu, L.; Meziani, M. J.; Wang, X.; Luo, P. G.; Veca, L. M.; Cao, L.; Sun, Y.-P. Advances in bioapplications of carbon nanotubes. Adv. Mater. 2009, 21, 139−152. (9) Girifalco, L. A.; Hodak, M.; Lee, R. S. Carbon nanotubes, buckyballs, ropes, and a universal graphitic potential. Phys. Rev. B: Condens. Matter Mater. Phys. 2000, 62, 13104−13110. (10) Backes, C. Noncovalent Functionalization of Carbon Nanotubes; Springer: Berlin, 2012. (11) Blanch, A. J.; Lenehan, C. E.; Quinton, J. S. Parametric analysis of sonication and centrifugation variables for dispersion of single walled carbon nanotubes in aqueous solutions of sodium dodecylbenzene sulfonate. Carbon 2011, 49, 5213−5228. (12) Fernandes, R. M. F.; Abreu, B.; Claro, B.; Buzaglo, M.; Regev, O.; Furó, I.; Marques, E. F. Dispersing carbon nanotubes with ionic surfactants under controlled conditions: Comparisons and insight. Langmuir 2015, 31, 10955−10965. (13) Haggenmueller, R.; Rahatekar, S. S.; Fagan, J. A.; Chun, J.; Becker, M. L.; Naik, R. R.; Krauss, T.; Carlson, L.; Kadla, J. F.; Trulove, P. C.; et al. Comparison of the Quality of Aqueous Dispersions of Single wall carbon nanotubes using surfactants and biomolecules. Langmuir 2008, 24, 5070−5078. (14) Islam, M. F.; Rojas, E.; Bergey, D. M.; Johnson, A. T.; Yodh, A. G. High weight fraction surfactant solubilization of single-wall carbon nanotubes in water. Nano Lett. 2003, 3, 269−273. (15) O’Connell, M. J.; Bachilo, S. M.; Huffman, C. B.; Moore, V. C.; Strano, M. S.; Haroz, E. H.; Rialon, K. L.; Boul, P. J.; Noon, W. H.; Kittrell, C.; et al. Band gap fluorescence from individual single-walled carbon nanotubes. Science 2002, 297, 593−596. (16) Zheng, M.; Jagota, A.; Semke, E. D.; Diner, B. A.; Mclean, R. S.; Lustig, S. R.; Richardson, R. E.; Tassi, N. G. DNA-assisted dispersion and separation of carbon nanotubes. Nat. Mater. 2003, 2, 338−342. (17) Shin, H.; Min, B. G.; Jeong, W.; Park, C. Amphiphilic block copolymer micelles: new dispersant for single wall carbon nanotubes. Macromol. Rapid Commun. 2005, 26, 1451−1457. (18) Bonnet, P.; Sireude, D.; Garnier, B.; Chauvet, O. Thermal properties and percolation in carbon nanotube-polymer composites. Appl. Phys. Lett. 2007, 91, 201910. (19) Star, A.; Stoddart, J. F.; Steuerman, D.; Diehl, M.; Boukai, A.; Wong, E. W.; Yang, X.; Chung, S.-W.; Choi, H.; Heath, J. R. Preparation and properties of polymer-wrapped single-walled carbon nanotubes. Angew. Chem., Int. Ed. 2001, 40, 1721−1725. (20) Cherukuri, P.; Gannon, C. J.; Leeuw, T. K.; Schmidt, H. K.; Smalley, R. E.; Curley, S. A.; Weisman, R. B. Mammalian pharmacokinetics of carbon nanotubes using intrinsic near-infrared fluorescence. Proc. Natl. Acad. Sci. U. S. A. 2006, 103, 18882−18886. (21) Bandyopadhyaya, R.; Nativ-Roth, E.; Regev, O.; YerushalmiRozen, R. Stabilization of individual carbon nanotubes in aqueous solutions. Nano Lett. 2002, 2, 25−28. (22) Wang, S.-F.; Shen, L.; Zhang, W.-D.; Tong, Y.-J. Preparation and mechanical properties of chitosan/carbon nanotubes composites. Biomacromolecules 2005, 6, 3067−3072. (23) Minami, N.; Kim, Y.; Miyashita, K.; Kazaoui, S.; Nalini, B. Cellulose derivatives as excellent dispersants for single-wall carbon nanotubes as demonstrated by absorption and photoluminescence spectroscopy. Appl. Phys. Lett. 2006, 88, 093123. (24) Riou, I.; Bertoncini, P.; Bizot, H.; Mevellec, J. Y.; Buléon, A.; Chauvet, O. Carboxymethylcellulose/single walled carbon nanotube complexes. J. Nanosci. Nanotechnol. 2009, 9, 6176−6180. (25) Revol, J. F.; Bradford, H.; Giasson, J.; Marchessault, R. H.; Gray, D. G. Helicoidal self-ordering of cellulose microfibrils in aqueous suspension. Int. J. Biol. Macromol. 1992, 14, 170−172. (26) Elazzouzi-Hafraoui, S.; Nishiyama, Y.; Putaux, J.-L.; Heux, L.; Dubreuil, F.; Rochas, C. The shape and size distribution of crystalline nanoparticles prepared by acid hydrolysis of native cellulose. Biomacromolecules 2008, 9, 57−65. (27) Eichhorn, S. J. Cellulose nanowhiskers: promising materials for advanced applications. Soft Matter 2011, 7, 303−315.

role in the dispersion process. A minimum power density is required to initiate the dispersion, which is higher for SWNTs than for MWNTs. Conversely, the association between CNCs and nanotubes is faster for MWNTs than for SWNTs. These results can be explained by their different initial state, bundling versus entanglement for SWNTs and MWNTs, respectively. AFM images and TEM micrographs show that hybrids between CNCs and nanotubes are formed. We argue that the association between both nano-objects is due to hydrophobic interactions between the hydrophobic (200) crystalline planes of the CNCs and the highly hydrophobic graphitic surface of the nanotubes, leading to the alignment of the CNCs along the tube axis in the SWNT/CNC hybrids. Such an alignment is not observed for MWNT/CNC hybrids. A stoichiometric model is proposed to fit the dependence of the dispersion yield with the initial concentration of CNCs and nanotubes. Surprisingly, and despite the difference in morphology of the hybrids, we found that the mass ratio of the nanotube concentration to the CNC concentration is ∼5% for both types of nanotubes. This very simple model allows us to predict the dispersion yield and thus the final CNTs concentration. Thus biobased CNCs appear to be very efficient to prepare aqueous dispersions of SWNTs or MWNTs in a noncovalent and surfactant-free way. It may open the way to biomedical applications.

■

ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jpcc.6b07289. Calculations of extinction coefficients for both SWNT/ CNC and MWNT/CNC hybrids and FTIR spectra of the CNC and the hybrids are provided. (PDF)

■

AUTHOR INFORMATION

Corresponding Author

*Tel: +33 (0)2 40 37 39 82. E-mail: Olivier.chauvet@cnrs-imn. fr. Notes

The authors declare no competing financial interest.

■

ACKNOWLEDGMENTS We gratefully acknowledge Emilie Perrin and BIBS platform from INRA (Institut National de la Recherche Agronomique) for TEM pictures. This work was supported by the University of Nantes and the French Ministry of Research.

■

REFERENCES

(1) Iijima, S. Helical microtubules of graphitic carbon. Nature 1991, 354, 56−58. (2) Monthioux, M.; Kuznetsov, V. L. Who should be given the credit for the discovery of carbon nanotubes? Carbon 2006, 44, 1621−1623. (3) Popov, V. N. Carbon nanotubes: properties and application. Mater. Sci. Eng., R 2004, 43, 61−102. (4) Dresselhaus, M. S.; Dresselhaus, G.; Eklund, P. C. Science of Fullerenes and Carbon Nanotubes; Academic Press: San Diego, 1996. (5) Bachilo, S. M.; Strano, M. S.; Kittrell, C.; Hauge, R. H.; Smalley, R. E.; Weisman, R. B. Structure-assigned optical spectra of singlewalled carbon nanotubes. Science 2002, 298, 2361−2366. (6) De Volder, M. F. L.; Tawfick, S. H.; Baughman, R. H.; Hart, A. J. Carbon nanotubes: Present and future commercial applications. Science 2013, 339, 535−539. G


Article

The Journal of Physical Chemistry C (28) Mazeau, K. On the external morphology of native cellulose microfibrils. Carbohydr. Polym. 2011, 84, 524−532. (29) Kalashnikova, I.; Bizot, H.; Cathala, B.; Capron, I. Modulation of cellulose nanocrystals amphiphilic properties to stabilize oil/water interface. Biomacromolecules 2012, 13, 267−275. (30) Olivier, C.; Moreau, C.; Bertoncini, P.; Bizot, H.; Chauvet, O.; Cathala, B. Cellulose Nanocrystal-assisted dispersion of luminescent single-walled carbon nanotubes for layer-by-layer assembled hybrid thin films. Langmuir 2012, 28, 12463−12471. (31) Horcas, I.; Fernández, R.; Gómez-Rodríguez, J. M.; Colchero, J.; Gómez-Herrero, J.; Baro, A. M. WSXM: A software for scanning probe microscopy and a tool for nanotechnology. Rev. Sci. Instrum. 2007, 78, 013705. (32) Araki, J.; Wada, M.; Kuga, S.; Okano, T. Flow properties of microcrystalline cellulose suspension prepared by acid treatment of native cellulose. Colloids Surf., A 1998, 142, 75−82. (33) Huang, Y. Y.; Terentjev, E. M. Dispersion of carbon nanotubes: Mixing, sonication, stabilization, and composite properties. Polymers 2012, 4, 275−295. (34) Lucas, A.; Zakri, C.; Maugey, M.; Pasquali, M.; van der Schoot, P.; Poulin, P. Kinetics of nanotube and microfiber scission under sonication. J. Phys. Chem. C 2009, 113, 20599−20605. (35) Adewuyi, Y. G. Sonochemistry: Environmental science and engineering applications. Ind. Eng. Chem. Res. 2001, 40, 4681−4715. (36) Strano, M. S.; Moore, V. C.; Miller, M. K.; Allen, M. J.; Haroz, E. H.; Kittrell, C.; Hauge, R. H.; Smalley, R. E. The role of surfactant adsorption during ultrasonication in the dispersion of single-walled carbon nanotubes. J. Nanosci. Nanotechnol. 2003, 3, 81−86. (37) Laurent, C.; Flahaut, E.; Peigney, A. The weight and density of carbon nanotubes versus the number of walls and diameter. Carbon 2010, 48, 2994−2996. Peigney, A.; Laurent, C.; Flahaut, E.; Bacsa, R. R.; Rousset, A. Specific surface area of carbon nanotubes and bundles of carbon nanotubes. Carbon 2001, 39, 507−514. (38) The surface density of the graphene sheet, 7.6 × 10−7 kg·m−2, is obtained by dividing the mass of two carbon atoms by the surface of the hexagonal unit cell of graphene.

H


Highly Efficient and Predictable Noncovalent ... - ACS Publications

Recommend Documents