Transitions of Aggregation States for Concentrated Carbon Nanotube

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Transitions of Aggregation States for Concentrated Carbon Nanotube Dispersion Takashi Hiroi, Seisuke Ata, and Mitsuhiro Shibayama J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.5b12464 • Publication Date (Web): 26 Feb 2016 Downloaded from http://pubs.acs.org on March 3, 2016

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The Journal of Physical Chemistry

To be submitted to The Journal of Physical Chemistry C Feb. 25, 2016

Transitions of Aggregation States for Concentrated Carbon Nanotube Dispersion

Takashi Hiroi,*1 Seisuke Ata,2 and Mitsuhiro Shibayama*3

1

Department of Chemistry, Graduate School of Science, The University of Tokyo, 7-3-1

Hongo, Bunkyo-ku, Tokyo 113-0033, Japan 2

National Institute of Advanced Industrial Science and Technology (AIST), 1-1-1

Higashi, Tsukuba, Ibaraki 305-8565, Japan 3

Institute for Solid State Physics, The University of Tokyo, 5-1-5 Kashiwanoha,

Kashiwa, Chiba 277-8581, Japan * To whom correspondence should be addressed.

*E-mail [email protected]; [email protected]

1 ACS Paragon Plus Environment


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Abstract Because of the lack of appropriate techniques for the measurement of concentrated dispersions, dispersion states of carbon nanotube (CNT) dispersions have been evaluated for dilute dispersions by assuming the dispersion state being unchanged by dilution. In this paper, it is clarified that this assumption does not hold true at high concentration region by a direct measurement of size distribution and anisotropy for CNT dispersions in wide concentration region. CNT dispersions showed a dispersion-state transition as a form of rotation restriction at certain concentration. In addition to this, CNT dispersions whose solutes have large specific surface area showed another dispersion-state transition at certain concentration as a form of aggregation growth. To prove these dispersion-state transitions from another point of view, the difference in sheet resistance of conducting layers made from different CNT dispersions coated on a glass substrate was investigated. It was confirmed that their sheet resistance also showed clear difference. This difference can be explained from the viewpoint of dispersion-state transitions induced by the drying process.


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Introduction The superior mechanical, electrical, and thermal properties of carbon nanotubes (CNTs) and their high aspect ratios make them attractive fillers for composite materials. The outstanding properties of CNTs have been utilized to develop multifunctional composites for several applications in the fields of chemical sensing,1 electrical and thermal management,2 photoemission,3 stretchable electro-device,4

5

electromagnetic shielding,6 7 energy storage performance8 and so on. As a source of the composite materials, CNTs are often dispersed in certain solvents, especially in water. Therefore CNT dispersion state has been recognized as one of the key factors to bring out the latent capabilities of CNTs over the past decade. The dispersion state of CNTs is not so simple because of their strong tendency to bundle together through van der Waals interactions.9 To measure these dispersion states, there are mainly two kinds of approach. First approach is direct observation by using SEM, TEM or AFM.10

11

These techniques enable us to see not

only size distribution but also aspect ratio of CNT aggregates. One of the main drawbacks is that the sample dispersion should be dried up before the measurement. This process may change their dispersion states. Second approach is dynamic light scattering (DLS). By using this technique, size distribution of the dispersion is obtained by using the solution. By applying polarization-selective measurement, the aspect ratio is also estimated by DLS.12

13 14

However, the dispersion should be diluted sufficiently

enough before the measurement to suppress the effect of multiple scattering and light absorption. Therefore these two approaches have the same problem that the original dispersion cannot be measured as it is. There is another technique to track the dynamics



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of solutes called total internal reflection microscopy.15-16 However, the applicability of this technique is limited to a thin layer. Most of the experimental results have been interpreted under the assumption that there is no concentration dependence in dispersion state. However, this point should be examined experimentally for the deep understanding of CNT dispersions. If there are some kinds of dispersion-state transitions induced by concentration change, the dispersion state difference may affect the product made from the CNT dispersion. To clarify this point, it is necessary to develop a characterization technique for CNT dispersions without dilution. Instead of the two techniques described above, there are several reports to measure CNT dispersions without dilution by other techniques. For examples, rheological measurements are reported even for relatively high concentration region

17 18

scattering

though it is difficult to obtain detail structural information. Small-angle

19 20 21

is useful to obtain structural information. However, this technique is

not suitable for daily assessment of products. We have reported that an aggregation growth occurs in Chinese ink at high concentration by using an apparatus named DLS microscope.22 DLS microscope is a technique to obtain DLS signal for a sample on the stage of an inverted microscope. By using this apparatus, the optical length is reduced to the order of micrometer so that the effect of light absorption is negligible. Therefore concentrated dispersions can be measured without dilution. In this paper, dispersions made from various types of CNTs were investigated in wide concentration region by using DLS microscope. Clear dispersion-state transitions were observed as aggregation growth as well as rotation restriction at certain concentration. To see how this state transition affects the physical properties of CNT-containing product, conducting layers were prepared by drying up 4 ACS Paragon Plus Environment

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the CNT suspensions and their performance was evaluated. From these results, the importance of the dispersion-state transition was clarified.



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Experimental Section Materials and Sample Preparation. Two types of multi-walled carbon nanotubes (MW-CNT) and three types of single-walled carbon nanotubes (SW-CNT) were used: Nanocyl (NC7000, Nanocyl S.A.), VGCF-X (Showa Denko K.K.), HiPCO (purified, Unidyme Inc.), CoMoCAT (CG200, Southwest Nano Technologies), SG (AIST).23 The characteristics of these CNTs are given in Table 1. The CNTs were dispersed in water with sodium deoxycholate (SDOC, Tokyo Chemical Industry Co., LTD.) by cyclic probe sonicator (UX-050, Mitsui Electric CO., LTD.) at 50 W power and 20 kHz for 12 h. The original concentration of the CNT in the suspensions was 0.1 wt%. Concentrated suspensions were prepared by evaporation of solvent from the original suspension. These suspensions were then mixed with SDOC so that the final proportion of CNT and SDOC was 1 : 3 (ratio by weight). These mixtures were vortexed vigorously for five hours. After that, these suspensions were centrifugalized (6000 rpm) for five hours. Supernatant dispersions of these suspensions were used as samples.

TABLE 1: List of CNTs product

ID

single-walled /

average

average

specific surface

G/D

multi-walled

diameter / nm

length / µm

area / m2 g-1

ratio

NC7000

Nanocyl

Multi

9.5

1.5

~ 280

0.8

VGCF-X

VGCF-X

Multi

10-15

3.0

~ 250

0.9

CoMoCAT

CoMoCAT

Single

0.8

5.0

~ 450

~ 15

HiPCO

HiPCO

Single

0.8

1.0

~ 300*

~ 50

super growth

SG

Single

3.7

1200

~ 1150

~5

CNT

*Value obtained from literature.24 6 ACS Paragon Plus Environment

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UV-VIS Measurement. The concentrations of supernatant dispersion for each suspension were measured by UV-VIS spectrometer (V-630, JASCO, Japan). The thickness of the optical cell was 0.2 cm. The samples were diluted to set absorbance less than 2. After that, the absorbance of the original dispersion with the optical length being 1 cm was calculated by using Lambert-Beer law. All of the data are shown in Figure S1 and S2. Because of the existence of large aggregates, the measured absorbance was not proportional to the original concentration (See Result and Discussion, 1). By assuming that there were no large aggregates at the lowest concentration (1 mg mL-1), the proportion of large aggregates 𝑃 were calculated as follows: 𝑃 = 1 − !⋅!

!!"# (!) !"#

! !"⋅!"!!

(1)

where 𝑐 is the concentration of original suspensions (mg mL-1) and 𝐴!"# (𝑐) is the measured absorption of the supernatant dispersions whose original concentration was 𝑐. Though 𝑃 should be 0 < 𝑃 < 1, some samples show slightly negative 𝑃 at a low concentration region because of an experimental error. In these data, 𝑃 was set to be zero because the amount of large aggregates is less than the detection limit in these data.

AFM Measurement. AFM images were taken using a Bruker Dimension FastScan. Samples were prepared by spin-coating technique from 10 folds diluted CNT suspension (typically 0.01 wt% CNT suspension). The analysis for the distribution of CNT lengths was done by using ImageJ.25

DLS Measurement. Two DLS apparatuses were used for the measurement. One is a



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conventional DLS system (DLS/SLS 5000 compact goniometer, ALV, Germany, the wavelength λ = 632.8 nm, the scattering angle θ = 30 ~ 130°) and the other is homemade DLS microscope we have recently reported22 (λ = 514.5 nm, θ = 180°). For polarized or depolarized DLS by the conventional DLS system, a polarizer was inserted between the sample point and a detector. The angle of polarizer for VH (vertical and horizontal polarization for incident and scattered light, respectively) configuration was determined to minimize the scattering intensity from a polystyrene latex suspension. The angle for VV (vertical polarization for both incident and scattered light) configuration is then set by 90° rotation from VH configuration. Schematic of DLS microscope is shown in Figure S3. A different point from the previous report is the option for manipulation of polarization. A half-wave plate was inserted in front of the objective lens to determine the polarization direction of incident light. In addition to this, a polarizer was inserted between the objective lens and the detector to select VV configuration or VH configuration. For most of the measurements, the laser power at the sample point is set to less than 1 mW (milli Watt) to reduce effect of heat. To measure depolarized scattering, the intensity was increased up to several tens mW since VH scattering is very weak. Concerning the data of DLS microscope, the initial amplitudes of time correlation functions ( 𝑔(!) 𝑡 = 0 − 1 ) were set to 0.1 by multiplying appropriate constant for clarity (rationalization of this manipulation is given in Supporting Info, Figure S4).

Analysis for Polarized DLS. Depolarized DLS is a technique to analyze the difference of q-dependence for relaxation time between VV configuration and VH configuration. A previous research14 shows that the relaxation rates for VV and VH configurations for 8 ACS Paragon Plus Environment

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rod-like solutes are calculated as follows: 𝛤!! = 𝐷! 𝑞!

(1)

𝛤!" = 𝐷! 𝑞! + 6𝐷!

(2)

where q is the magnitude of scattering vector, Dt is the translational diffusion coefficient and Dr is the rotational diffusion coefficient. Dt and Dr are approximately represented by the following equations: ! !

! 𝐷! = !!!

𝐷! =

!!

!!! ! !!!! ! !

ln

!! !

! ! !

+ ln 2 − 1 ! ! ! !

!"

(3)

! ! !

! !

!"!!!" !!! !!.!"# !"!

(4)

where kB, T, ηs, a, b are Boltzmann constant, the absolute temperature, the viscosity of solvent, the half-length of solutes and radius of solutes, respectively. Here, Dt and Dr are functions of a and b. By using this relationship, the values of a and b are estimated from the relaxation rates obtained with VV and VH configurations. Note that an estimated value does not give quantitative meaning since this procedure uses a coarse approximation.

Preparation of Conducting Layers and Sheet Resistance Measurement. Conducting layers were prepared from CNT dispersions with different concentration to evaluate the effect of dispersion states. An oxygen plasma treatment was applied to glass slides to improve hydrophilicity of the surface. These slides were coated by spray coating method (SANKEI-Tech, 2200N-mini) and washed by ethanol. The concentration of CNT dispersions used for spray coating was 0.01 wt%. The final amount of CNT on the surface was set to 0.3 mg, 0.4 mg or 0.8 mg. Sheet resistance for each glass slide was measured by a resistivity meter (Mitsubishi Chemical Analytech, MCP-T610). 9 ACS Paragon Plus Environment


Results and Discussion 1. Characterization of Dispersions. Proportion of the large aggregates

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1.0 Nanocyl VGCF CoMoCAT HiPCO SG

0.8 0.6

} MW

}

SW

0.4 0.2 0.0 1

2

3

4 5 67

2

3

4 5 6

10 -1 Conc. of original suspension / mg mL

Figure 1. Concentration dependence of proportion of large aggregates for five kinds of CNT dispersions evaluated by the UV-VIS spectrometer. At low concentration, several points are overlapped at the bottom region (shown in a yellow band). Deviation points from this region are shown for each CNT as arrows.

Usual carbon nanotube dispersions contain large aggregates whose order is larger than 1 µm because of their strong aggregation tendency. The scattered light originated from these large aggregates is very strong since scattered intensity is proportional to the square of the volume of scatterers. In addition to this, theses large aggregates are not the interest in this work since they can be directly evaluated by usual microscopes. Therefore these large aggregates were removed by centrifugation before the DLS measurements. The proportion of these large aggregates to the total amount of solutes was evaluated by UV-VIS spectrometer. The results are shown in Figure 1 for


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five different CNT species. Specification for these five CNTs is summarized in Table 1. Figure 1 clearly shows that the proportion of the large aggregates starts increasing at a certain concentration of original suspensions (the arrows shown in Figure 1). At this threshold concentration, thermal energy becomes comparable to the aggregation tendency. This threshold value depends on CNT species; the threshold concentration of multi-walled CNT (MW-CNT) is smaller than that of single-walled CNT (SW-CNT). In the following discussion, the results for HiPCO and SG are used since the concentration changes induced by centrifugation of these two CNTs are relatively small in a wide concentration region of the original suspension compared to the other CNT species. The results for other three species are also shown in Supporting Information, Figure S5~7.



2. Determination of Size Distribution by DLS.

(a)

0.1 0.25 wt% 0.5 wt% 0.75 wt% 1 wt% 2 wt% 3 wt% 4 wt% 5 wt%

0.01

0.001 0.001

0.01

0.1

1

10

100

(b) 5 wt% 4 wt% 3 wt% 2 wt% 1 wt% 0.75 wt% 0.5 wt% 0.25 wt% 0.001 wt%

10

(c)

-9

10

-8

-7

10

10

-6

-5

10

0.6 0.5

Probability

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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0.4 0.3 0.2 0.1 0.0

0

20

40

60

80

100

Figure 2. Concentration dependence of (a) time correlation functions and (b) size distribution functions for HiPCO dispersions measured by DLS. Labels in the


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figure stands for the concentration of original suspension. (c) A size distribution function for HiPCO dispersions measured by AFM.

Figure 2(a) shows the concentration dependence of time correlation functions for HiPCO dispersions measured without dilution by DLS microscope. Note that some samples reached the absorbance up to 50 and beyond the scope of conventional DLS system (Figure S1). In the case of HiPCO, all of the time correlation functions show almost the same decay curve regardless of their concentrations. This point is also clarified in size distribution functions constructed from the time correlation functions (Figure 2(b)). In Figure 2(b), the size distribution function obtained from a very dilute dispersion (0.001 wt%) is also shown. This dilute dispersion is measured by the conventional DLS system since the concentration is too low to be measured by DLS microscope. Note that our previous paper proved that the same results are obtained from DLS microscope and conventional DLS system.22 Although there are small differences originated from the incompleteness of inverse Laplace transformation, all of the size distribution functions show a similar size distribution. Therefore it is concluded that the aggregation states of HiPCO do not show any concentration dependence. To clarify the novelty of the DLS measurement, the size distribution of the same sample was evaluated by AFM. The size distribution for HiPCO obtained from an AFM image is shown in Figure 2(c). The information obtained from AFM is the lengths of CNTs. In Figure 2(c), CNT lengths were converted into the hydrodynamic radius (the procedure is shown in Supporting Information, Figures S8 and S9).26 Apparently, the size distributions obtained from DLS and AFM are different; the hydrodynamic radius obtained from DLS is larger than that obtained from AFM. This discrepancy is



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originated from the difference of measurement principle. In the case of DLS, what we measure is the intensity correlation function of scattered light. Since the intensity of scattered light is proportional to the square of the volume of the scatterers, DLS has high sensitivity for larger scatterers. In contrast to this, AFM see the solutes directly. That is why the hydrodynamic radius obtained from DLS is larger than that obtained from AFM. In the case of monodisperse solutes, the size measured by DLS and AFM shows good agreement.14, 27 Note that the concentration dependence of size distribution functions cannot be obtained from AFM measurement since the samples for AFM is dried up. Therefore it should be avoided to evaluate the dispersion state at high concentration region by neither conventional DLS nor AFM.


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(a)

0.1

0.1 wt% 0.2 wt% 0.3 wt% 0.4 wt% 0.5 wt%

0.01

0.001 0.001

0.01

0.1

1

10

100

(b)

0.5 wt% 0.4 wt% 0.3 wt% 0.2 wt% 0.1 wt% 0.001 wt%

-9

10

(c)

10

-8

-7

10

10

-6

10

-5

0.6 0.5

Probability

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


0.4 0.3 0.2 0.1 0.0

0

20

40

60

80

100

Figure 3. Concentration dependence of (a) time correlation functions and (b) size distribution functions for SG dispersions measured by DLS. Labels in the figure stands for the concentration of original suspension. (c) A size distribution function



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for SG dispersions measured by AFM.

In contrast to the case of HiPCO, the aggregation states of SG show clear concentration dependence. Figure 3(a) shows the concentration dependence of time correlation functions for SG dispersions and Figure 3(b) is the corresponding size distribution functions. Decay of the time correlation function becomes slow as the concentration increases (shown by an arrow in Figure 3(a)). This slowing is originated from the aggregation growth (shown by a dashed square in Figure 3(b)). Similar aggregation growth in concentrated dispersion has also been reported in Chinese ink.22 This difference between HiPCO and SG is originated from the difference of specific surface area. Since SG has relatively large surface area (see Table 1), SG aggregates more easily by hydrophobic force than others, which acts between their surfaces. The size distribution for SG obtained from an AFM image is shown in Figures 3(c) and S10. Compared to the size distribution for HiPCO (Figure 2(c)), SG has longer shape although it is shorter than the original length (Table 1). The reason for this is that the SG CNTs were broken off during vortex. The reason for the difference of the size distribution obtained from DLS and AFM is similar to the case for HiPCO. For SG, it is proved later that the average length obtained from polarized DLS shows good agreement with the AFM result (next section). Other three types of CNTs were also investigated by the procedure explained here (Supporting Information, Figure S5~7). The aggregation states of Nanocyl and VGCF-X showed almost no concentration dependence, which is similar to the result of HiPCO. In contrast to this, the aggregation states of CoMoCAT show strong concentration dependence, which is similar to SG. The threshold concentration to start


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aggregation growth is higher in the case of CoMoCAT (1 ~ 2 wt%) than that of SG (0.2 ~ 0.3 wt%). These results can be explained by the difference of specific surface area. CNTs whose specific surface area is small (Nanocyl, VGCF-X, HiPCO) did not show any aggregation growth. In contrast to this, CNTs whose specific surface area is large (CoMoCAT, SG) clearly show aggregation growth. The difference of the threshold concentration is also explained by the specific surface area of CNT; the larger surface area is, the smaller the threshold concentration becomes.



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3. Estimation of Anisotropy by Polarized DLS.

(a)

1

0.1

PS VV VH

0.01

0.001 0.001

(b)

SG VV VH

0.01

0.1

1

10

0.01

0.1

1

10

100

0.01

0.1

1

10

0.01

0.1

1

10

100

0.1

0.01

0.001 0.001

Figure 4. Polarization dependence of representative time correlation functions for polystyrene latex suspension (PS, diameter: 100 nm) and SG dispersion. (a) Time correlation functions of (left) 0.001 wt % PS and (right) 0.001 wt % SG measured by conventional DLS system (θ = 50°). (b) Time correlation functions for (left) 0.1 wt % PS and (right) 0.1 wt % SG measured by DLS microscope. Solid line: VV configuration. Broken line: VH configuration.

Although DLS is a powerful technique to determine size distribution of solutes, it is difficult to obtain information about their shape. Not only the size but also


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the aspect ratio of CNTs strongly affects their physical properties. Therefore a simple, easy and non-destructive method for the estimation of radius and length of CNTs has been strongly desired. This point was partially solved by using a depolarized DLS technique.12 Depolarized DLS is a technique to obtain translational and rotational diffusion constants from time correlation functions of scattered light whose polarization is parallel or perpendicular to the incident light. The length and radius for rod-like solutes are calculated from their translational and rotational diffusion constants (details for the analysis procedure is shown in Materials and Methods section). To confirm this method works well to our system, depolarized DLS for a dilute polystyrene latex suspension and the CNT dispersion was measured by using conventional DLS system. SG was used as a representative of CNT dispersion by taking their high aspect ratio into consideration. A representative result is shown in Figure 4(a) (Data measured at different angles are shown in Figure S11). A clear difference appeared in the relaxation rates between VV and VH configurations for SG dispersion while polystyrene latex suspension (PS) does not show any difference. The reason why PS does not show significant difference between VV and VH configurations is that the rotational diffusion coefficient Dr = 0 for a = b where a and b are the half-length of solutes and the radius of solutes, respectively. From the calculation results for SG (the translational diffusion coefficient Dt = 3.6×10-12 m2 s-1, Dr = 1.1×102 s-1 calculated in Figure S12), the average size of this SG was calculated approximately as a = 300 nm and b = 7 nm. This data agrees well with the AFM analysis (shown in Figure S10). Then polarized DLS was applied for concentrated samples by using DLS microscope. Figure 4(b) shows the results for PS and SG. The concentrations of both dispersions are 100 times higher than that shown in Figure 4(a). Interestingly, the 19 ACS Paragon Plus Environment


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difference between relaxation rates of VV and VH configurations is not observed not only for PS but also for SG. The possible reason for this is the rotation restriction for each SG CNT solute occurred in relatively high concentration region. Since the size distributions of the SG dispersions are almost the same for 0.001 wt % and 0.1 wt% dispersions (Figure 3(b)), the disappearance of the difference of relaxation rates is not attributed to the change of the solute size induced by concentration change. In other words, Dts of the SG dispersion at 0.001 wt% and 0.1 wt% dispersion are the same. Therefore it is concluded that Dr of the SG dispersion at 0.1 wt% is zero. The physical meaning for Dr = 0 is that the solute does not rotate in the solution. This rotation restriction is rationalized by taking large aspect ratio of SG CNT solutes into consideration.


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4. Dispersion-state transition and its effect on physical properties.

Low

Concentration

High

Figure 5. Proposed concentration dependence for aggregation state of CNT dispersions.

From these experimental results, we propose a model for dispersion-state transitions induced by concentration change as shown in Figure 5. In terms of low concentration region (~ 0.001 wt%), the anisotropic nature of CNT dispersions is clearly shown from polarized DLS measurement performed by the conventional DLS system (Figure 4(a)). This means that each solute shows translational and rotational Brownian motion. However, at relatively high concentration region (~ 0.1 wt%), the rotational Brownian motion is restricted as explained in the previous section (Figure 4(b)). Note that the form of each solute is almost the same as that of dilute one in this transition (Figure 2(b) and 3(b)). For SG, there is another dispersion-state transition as a form of aggregation growth as shown in Figure 3. Whether this aggregation growth occurs or not is related to the magnitude of the specific surface area.



5

sq

-1

10

4

Sheet resistance /

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10

3

10

2

10

HiPCO

0.4 mg

0.8 mg

0.3 mg

SG

0.8 mg

Amounts of CNTs on the surface of glass slides

Figure 6. Sheet resistance for glass substrates coated by HiPCO or SG. The final amounts of CNTs on the surface of glass slides are set to 0.3 mg, 0.4 mg or 0.8 mg.

To see how the proposed model affects the physical properties of CNTs, characterization of CNT conducting layers was performed. It is well known that CNTs are good conducting material, which is originated from π electron clouds around CNTs. One of the applications of CNT is the material for conducting layer. To prepare CNT conducting layer, CNT dispersions are sprayed to substrate and dried up. Therefore the dispersion-state transitions induced by the drying process may affect the conductivity of the resultant conducting layers. To confirm this point, CNT conducting layers were prepared from HiPCO and SG dispersions. Figure 6 shows the sheet resistance of conducting layers made from HiPCO and SG dispersions. In the case of HiPCO, the sheet resistance was decreased significantly by increasing the amount of CNTs on the surface (the arrow shown in Figure 6). In contrast to this, the sheet resistance of SG substrate did not depend on the amount of CNTs. In addition to this, the sheet resistance of SG substrate is smaller than


Page 23 of 37

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that of HiPCO substrate. These results are explained by taking the dispersion-state transitions into consideration. Schematics for the explanation are summarized in Figure 7. In the case of the HiPCO dispersions, the aggregation growth does not occur during the drying process. Because of this, the number of contacts among each solute is not so large. This fact is reflected as the relatively high sheet resistance. By increasing the amount of CNT on the surface, sheet resistance becomes small because the number of contacts among each solute was increased. In the case of the SG dispersions, the rotation restriction and the aggregation growth occurred during the drying process. Thanks to the aggregation growth of CNT solutes, SG solutes percolate at relatively low concentration. This consideration is consistent with the fact that the sheet resistance of SG substrate did not depend on the amount of CNTs if the percolation threshold is smaller than 0.3 mg for the current preparation condition. These results clearly show the importance of the dispersion-state transition for the application of CNT dispersions.



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Low

Concentration

Page 24 of 37

High

HiPCO

SG

Figure 7. Schematics of the surface of glass substrates made from HiPCO or SG dispersions with different concentrations.


Page 25 of 37

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Conclusions By using DLS microscope, the size distributions and anisotropy of CNT dispersions were measured in wide concentration region (0.001 wt% ~ 5 wt%). Two kinds of dispersion-state transitions were successfully observed. The first transition is aggregation-growth transition at high concentration region for CNTs whose specific surface area is large. The second transition is rotation-restriction transition for CNTs with high aspect ratio. Effect of the dispersion state on physical properties of CNT products was shown by using CNT conducting layers. CNT conducting layers show lower conductivity when the rotation restriction and the aggregation growth occur during the drying process. This result clearly shows that the dispersion-state transition affects the physical properties of CNT products.

Acknowledgement. The authors thank A. Nishizawa for her assistance of sample preparation. The authors also thank S. Sakurai and J. He for their measurement of AFM. This work has been financially supported by Grants-in-Aid for Scientific Research from the Ministry of Education, Culture, Sports, Science, and Technology (No. 25248027 to M.S.).

Supporting Information. Details of analysis and additional experimental data for different CNT species are shown in Supporting Information. This information is available free of charge via the Internet at http://pubs.acs.org.



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Page 26 of 37

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Table of Contents

Low

Concentration

High


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Proportion of the large aggregates


1.0 Nanocyl VGCF CoMoCAT HiPCO SG

0.8 0.6

Page 30 of 37

} MW

}

SW

0.4 0.2 0.0 1

2

3

4 5 67

2

3

4 5 6

10 -1 Conc. of original suspension / mg mL

ACS Paragon Plus Environment

Page 31 of 37

(a)

0.1 0.25 wt% 0.5 wt% 0.75 wt% 1 wt% 2 wt% 3 wt% 4 wt% 5 wt%

0.01

0.001 0.001

0.01

0.1

1

10

100

(b) 5 wt% 4 wt% 3 wt% 2 wt% 1 wt% 0.75 wt% 0.5 wt% 0.25 wt% 0.001 wt%

-9

10

(c)

10

-8

-7

10

10

-6

-5

10

0.6 0.5

Probability

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


0.4 0.3 0.2 0.1 0.0

0

20

40

60

80


100


(a)

0.1

0.1 wt% 0.2 wt% 0.3 wt% 0.4 wt% 0.5 wt%

0.01

0.001 0.001

0.01

0.1

1

10

100

(b)

0.5 wt% 0.4 wt% 0.3 wt% 0.2 wt% 0.1 wt% 0.001 wt%

-9

-8

10

(c)

-7

10

-6

10

10

10

0.6 0.5

Probability

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 32 of 37

0.4 0.3 0.2 0.1 0.0

0

20

40

60

80


100

-5

Page 33 of 37

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


(a)

1

0.1

0.01

0.001 0.001

(b)

PS VV VH

SG VV VH

0.01

0.1

1

10

0.01

0.1

1

10

100

0.01

0.1

1

10

0.01

0.1

1

10

100

0.1

0.01

0.001 0.001



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Low

Concentration


Page 34 of 37

High

Page 35 of 37

5

sq

-1

10

4

Sheet resistance /

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


10

3

10

2

10

HiPCO

0.4 mg

0.8 mg

0.3 mg

SG

0.8 mg

Amounts of CNTs on the surface of glass slides



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Low

Concentration HiPCO

SG


High

Page 36 of 37

Page 37 of 37

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Low

Concentration

High


Transitions of Aggregation States for Concentrated Carbon Nanotube

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