Optimizing sonication parameters for dispersion of single-walled carbon nanotubes

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Optimizing sonication parameters for dispersion of single-walled carbon nanotubes

Document Outline

(A)

(B)

Fig. 1.

UV–vis-NIR absorption spectra for CoMoCat SWCNTs dispersed in 1 w/v% DOC solution as a function of sonication power under a sonication time of 60 min (A); and as

a function of sonication time under a sonication power of 100 W (B). The initial concentration of CoMoCat SWCNTs was 0.025 w/v% for all samples.

H. Yu et al. / Chemical Physics 408 (2012) 11–16

these spectra changes can be attributed to the increase in the con-

centration of the individual SWCNTs and the small-diameter

SWCNT bundles in the solution. This implies that more SWCNTs

have been transferred from the larger-diameter bundles into the

smaller-diameter bundles and individual nanotubes after intense

sonication was applied.

The typical absorption spectra are composed of the resonant

absorption peaks and the nonresonant background, which reﬂects

the concentration of SWCNTs, the degree of nanotube debundling

and the nanotube lengths

[29]

. As shown in

Fig. 2

, we deﬁne the

resonant absorption area (A

), the nonresonant background area

) and the maximum absorption intensity (I

MAX

). In general,

the solution of SWCNTs includes a variety of (n, m) species, which

can lead to absorption peaks at different wavelengths. To evaluate

the dispersion quality, only several peaks with high intensities

were needed. With respect to the overall absorption spectra, fea-

tures of (6, 5) SWCNTs were picked up and discussed in detail. In

contrast to the S

peak, the S

displays a higher intensity associ-

ated with the concentration of SWCNTs and thereby the S

peak

was subsequently analyzed. Besides, there are another two absorp-

tion peaks for (9, 1) and (7, 5) SWCNTs, which can be seen at

915 nm and 1026 nm, respectively

[28,30]

. As a result, the aggrega-

tion of speciﬁc spectra features led to the broadening of the

absorption peaks

[30–31]

In this case, the peak group of S

(centered at

1000 nm) was

used as an approximation to analyze the absorption spectra. Thus

the difference in absorption spectra induced by the various sonica-

tion conditions can be quantiﬁed through the I

MAX

-to-FWHM ratio,

and A

ratio. For a given absorption spectrum of SWCNTs, it

has been shown that well-resolved features mean the fraction of

individual SWCNTs is higher

[29]

. The I

MAX

-to-FWHM ratio has

been proven to be related to the degree of SWCNT debundling

and aggregation, which is used to evaluate the purity of SWCNT

samples

[31–32]

. As shown in

Fig. 3

, we use the I

MAX

-to-FWHM ra-

tio to identify the dispersion state of SWCNTs in 1 w/v% DOC

solution.

An increase in this ratio as a function of the sonication time can

be attributed to an improvement in the degree of debundling of the

SWCNTs. The increase in sonication time caused a sharpening of

the optical absorption peaks and thus enhanced the resolution of

the spectra from the different SWCNT species. Similarly, the in-

crease in sonication power shows the trend towards reducing of

the amount of SWCNT bundles for a given sonication time. Inter-

estingly, for applied sonication powers from 20 and 40 W, the I

MAX

-

to-FWHM ratio appears to be proportional to the sonication time.

This illustrates that the solution mostly contained bundles of

Fig. 2.

Schematic of UV–vis-NIR absorption spectrum for CoMoCat SWCNTs.

100

120

0.005

0.01

0.015

0.02

0.025

0.03

0.035

0.04

0.045

Sonication time (min)

Max

−to−FWHM ratio (ABS/nm)

20W

40W

60W

80W

100W

120W

Fig. 3.

MAX

-to-FWHM ratio of the S

optical absorption peak as a function of

sonication time with a variety of sonication powers.

100

120

100

120

Sonication time (min)

Sonication power (W)

Resonance area

(A)

100

120

100

120

0.4

0.5

0.6

0.7

Sonication time (min)

Sonication power (W)

Resonance ratio

(B)

Fig. 4.

3-D mappings displaying (A) Resonance area (A

) and (B) Resonance ratio of the resonant area to the nonresonant area (A

ratio) as a function of the sonication

power and the sonication time along the x and y axis, respectively.

Table 1

The effects of SWCNT parameters on the optical absorption spectra.

Resonant Absorption

Background

Long

?

Short

;

Bundle

Individual

"

⁄

Low Con.

High Con.

⁄

Low C: means low concentration of SWCNTs; High C: means high concentration of

SWCNTs.

H. Yu et al. / Chemical Physics 408 (2012) 11–16

SWCNTs. In contrast, when the power was varied from 60 to

120 W, the I

MAX

-to-FWHM ratio shows an exponential dependence

on the sonication time. For sonication powers greater than 80 W,

the trend in the I

MAX

-to-FWHM ratio appears to be reaching a max-

imum value with increasing the sonication time. This can be ex-

plained because the resolution of the absorption peaks from

different (n, m) of SWCNTs improved continuously until the bun-

dles were totally dispersed.

Fig. 4

, A and B show the three-dimensional (3-D) mappings of

the A

and the A

ratio as a function of the sonication power

and the time duration, respectively. The resonant optical absorp-

tion depends on the concentration and the average length of iso-

lated SWCNTs, and thus can be used to characterize the degree of

debundling of the SWCNTs in the solution

[25,29,33]

. The initial

concentration of SWCNTs was the same, therefore the changes

are due to the various sonication parameters. The dispersion of

SWCNTs during sonication can be described as two processes of

debundling and shortening. First, it appears that the debundling

dominates the dispersion of the SWCNTs. In this case, the size of

the SWCNT bundles decreased remarkably and the isolated

SWCNTs increased gradually. After that, the debundling appears

not to be predominant and there are more individual nanotubes

than small-diameter bundles. Eventually, the shortening process

dominates the dispersion of the SWCNTs. As can be seen in

Fig. 4

A, A

shows an approximately exponential dependence on

the sonication time for a given power. It can be inferred that the

reaches the maximum value with the increase in sonication

time. The effect of sonication power shows the same trend for a gi-

ven sonication time. However, for a lower sonication power, it ap-

pears to be difﬁcult to obtain good dispersion of SWCNTs. This is

because the low mechanical energy produced by the tip or bath

sonicator cannot overcome the interaction of small-sized nanotube

bundles.

Nevertheless, it should also be noted that the well-dispersed

SWCNT solution still contains a few small-diameter bundles. To

investigate the debundling state of the SWCNTs, we compare the

R

/A

ratio of spectra from samples prepared under the various

sonication conditions. We ﬁrst consider the nonresonant back-

ground induced by the SWCNTs and other contaminants. Amor-

phous carbon and catalysts, as well as large-diameter bundles of

SWCNTs, have been removed from the solution. Therefore, the

nonresonant background mainly depends on the small-diameter

SWCNT bundles and the individual SWCNTs. A previous study

showed that a change in the length of the SWCNTs does not affect

the nonresonant background

[31]

. In contrast, the resonant absorp-

tion shows a strong dependence on the length of the SWCNTs. The

long individual SWCNTs can contribute to the high optical absorp-

tion intensity

[33]

. The relative effects of the length, degree of deb-

undling and the concentration of SWCNTs on the resonant

absorption and the nonresonant backgroundis shown in

Table 1

As can be seen in

Fig. 4

B, the ratio goes up to a maximum and

then declines with the sonication time for the given powers of

80, 100 and 120 W. It reﬂects the variation in SWCNT size during

the different dispersing phases. We suggest that most of SWCNT

bundles have been separated into the individual SWCNTs when

the ratio reaches its maximum. For the lower sonication power,

it shows that the solution contains large numbers of SWCNT bun-

dles. These bundles contribute to weaken the absorption intensity.

To investigate the effect of centrifugation on the absorption spectra

of the SWCNTs, we compare the spectra before and after 2 h of cen-

trifugation.

Fig. 5

A shows that the centrifugation has an important

inﬂuence on the background, which can be attributed to the re-

moval of the amorphous carbon, catalyst impurities and large-

diameter bundles from the SWCNT solution. By contrast, the

changes in the optical absorption are not obvious, which indicates

that the amorphous carbon has no contribution to the optical

absorption measurement.

Fig. 5

B shows the differences in the opti-

cal absorption before and after centrifugation.

The excess background rises obviously with the increase in the

sonication power, which results from the decreased size of the

amorphous carbon. In particular, the S

and S

peaks can be seen

for the sonication powers at 20, 40 and 60 W, which are related to

the large-diameter bundles removed from the solution during cen-

trifugation. This reveals that the SWCNTs have not been dispersed

adequately. Comparing the 40 W results with the 20 W data, the

Fig. 5.

UV–vis-NIR absorption spectra before and after centrifugation (A). Subtracted absorption induced by the amorphous carbon, catalyst impurities and large-diameter

bundles (B). The inset shows the subtracted absorption intensity of the S

peaks resulting from the various sonication powers.

Fig. 6.

Comparison of the absorption spectra for the SWCNT solution dispersed

using 3-mm and 6-mm sonication tips.

H. Yu et al. / Chemical Physics 408 (2012) 11–16

Fig. 7.

Tapping-mode AFM height images (5

l

m total area). Dispersion of CoMoCat SWCNTs sonicated under a power of 120 W with durations of 30 min (A), 60 min (B),

90 min (C) and 120 min (D), respectively (scale bar = 1

m).

0.4 0.6 0.8 1.0 1.2 1.4 1.6 1.8 2.0 2.2 2.4

Count (

)

Diameter (nm)

120 min

0.4 0.6 0.8 1.0 1.2 1.4 1.6 1.8 2.0 2.2 2.4

Count (%)

90 min

(A)

0.4 0.6 0.8 1.0 1.2 1.4 1.6 1.8 2.0 2.2 2.4

5

10

Count (%)

60 min

0.4 0.6 0.8 1.0 1.2 1.4 1.6 1.8 2.0 2.2 2.4

Count (%)

30 min

0.2 0.4 0.6 0.8 1.0 1.2 1.4 1.6 1.8 2.0 2.2 2.4

10

20

Count (%)

Length (

120 min

0.2 0.4 0.6 0.8 1.0 1.2 1.4 1.6 1.8 2.0 2.2 2.4

Count (%)

90 min

0.2 0.4 0.6 0.8 1.0 1.2 1.4 1.6 1.8 2.0 2.2 2.4

5

10

Count (%)

60 min

0.2 0.4 0.6 0.8 1.0 1.2 1.4 1.6 1.8 2.0 2.2 2.4

5

10

30

(B)

Count (%)

30 min

Fig. 8.

Histograms analyzing the diameters of more than 50 measured, individual SWCNTs. (A) Diameter statistics; (B) Length statistics. These SWCNTs were dispersed under

a power of 120 W with durations of 30, 60, 90 and 120 min, respectively.

H. Yu et al. / Chemical Physics 408 (2012) 11–16

amount of SWCNT bundles that is removed is low. The loss of

absorption intensity is due to their smaller sizes. This proves that

decreasing size of the SWCNT bundles results in increasing the

optical absorption. Moreover, tiny red shifts of

6 nm can be seen

with the increase insonication power. This denotes that different

(n, m) of the SWCNTs are sufﬁciently separated in the well-dis-

persed solution under the higher sonication power. And most of

the larger diameter SWCNTs congregated towards the bottom of

centrifuge tube due to the higher sedimentation rate.

To investigate whether the sonication tip has an inﬂuence on the

dispersion of the SWCNTs, we used a 6 mm tip (KE 76) to repeat the

dispersion. The test procedure is the same as the one described in

Section

. As illustrated in

Fig. 6

, the optical absorption spectra

resulted from the 6 mm of tip is much higher than that from the

3 mm of tip, under the same sonication powers. This suggests that

the same sonication power may result in difference in dispersion of

SWCNTs. It is because the larger diameter tip has a larger contact

area with the liquid which can transfer more energy and enable

the SWCNTs to overcome their interaction with each other.

For characterizing the diameter and length of the samples, AFM

measurements were carried out, as shown in

Fig. 7

. In particular,

the diameter refers to the height of the individual nanotube or

bundled nanotubes, which were measured from AFM images at dif-

ferent heights. Here we compare four samples of CoMoCat SWCNTs

dispersed with different sonication times of 30, 60, 90 and 120 min

at 120 W of sonication power using the 3-mm sonication tip.

The statistics for the diameters and lengths of the individual or

bundled nanotubes are shown in

Fig. 8

. The differences in the dis-

tribution of nanotube diameters as a function of the sonication

duration can be attributed to the strong dependence of the disper-

sion quality on the sonication process. In particular, the sonication

led to a reduction of nanotube length and thereby resulted in dra-

matically decreasing the interaction between the SWCNTs. As a re-

sult, the distribution of nanotube lengths appears to be a function

of the sonication time. Correspondingly, the inﬂuence of sonication

power on nanotube diameter and length shows a similar tendency

with time. It was found that most of the large-diameter bundles

were removed from the solution after centrifugation.

4. Conclusion

In summary, we have investigated the dispersion of CoMoCat

SWCNTs in a 1 w/v% DOC solution with a variety of sonication

parameters. The results show that an aqueous solution of SWCNTs

can be produced under each sonication parameter. However, the

quality of the dispersion depends on the sonication time and

power. Comparing to the soniﬁcation time, the sonication power

is found to be the more inﬂuential parameter on the subsequent

dispersion, showing that intense sonication is necessary for dis-

persing the smaller diameter CoMoCat SWCNTs. For a well-dis-

persed solution of SWCNTs, increasing the sonication time leads

to a shortening of the SWCNT length. Ultracentrifugation is effec-

tive for removing large-diameter bundles from the SWCNT solu-

tion and for achieving a homogenous solution. The sonication tip

with a larger diameter was discovered to perform better for disper-

sion of SWCNTs than the sonication tip with a smaller diameter.

Acknowledgment

The authors wish to thank Prof. O. Kanoun for the use of the

Lambda 900 spectrometer and Dr. M. Arnold and T. N. Dinh for

their help with UV-vis-NIR measurements. This work was ﬁnan-

cially supported by the German Research Foundation (DFG) in

the Forscherguppe 1713 ‘‘Sensorische Mikro-und Nanosysteme.’’

Further we acknowledge the Volkswagenstiftung in the funding

initiative ‘‘Integration of Molecular Components in Functional

Macroscopic Systems.’’ H. B. Yu is also indebted to the support

from the Fraunhofer-CAS Doctoral Training Program.

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H. Yu et al. / Chemical Physics 408 (2012) 11–16

Document Outline

Optimizing sonication parameters for dispersion of single-walled carbon nanotubes
- 1 Introduction
- 2 Experimental section
- 3 Results and discussion
- 4 Conclusion
- Acknowledgment
- References

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