Morphology control of three-dimensional carbon nanotubemacrostructures fabricated using ice-templating method

Sang-Hoon Park • Kwang-Heon Kim •

Kwang Chul Roh • Kwang-Bum Kim

� Springer Science+Business Media New York 2013

Abstract 3-Dimensional (3D) carbon nanotube (CNT)

macrostructures with controlled morphologies were prepared

by the ice-templating method. In order to control the assem-

bling of the CNTs into 3D macrostructures, we systematically

investigated the parameters critical to the control of the mor-

phology of the 3D CNT macrostructures formed using the ice-

templating method. It was found that process parameters such

as the initial characteristics of the CNT suspension and its

freezing conditions significantly affected the morphologies of

the resulting CNT macrostructures. By adjusting the initial

characteristics of the suspension of CNTs and its freezing

conditions, we could fabricate not only regular 3D CNT mac-

rostructures that consisted of aligned lamellae but also those

that had a cellular structure. This is the first instance well-

aligned, cellular CNT macrostructures have been prepared

using the ice-templating method. Our approach can be used to

develop the ice-templating method as a technique for fabri-

cating 3D pore- and structure-controlled CNT macrostructures.

Keywords 3-Dimensional (3D) macrostructures � Carbon

nanotubes � Lamellar macrostructures � Cellular

macrostructures � Ice-templating method

1 Introduction

Since their discovery, carbon nanotubes (CNTs) have

attracted significant attention worldwide owing to their

advantageous characteristics such as their exceptional

electrical and thermal conductivity, mechanical stability,

and their ability to form interconnected porous networks

with high surface areas [1–4].

The ability to assemble CNTs into macrostructures that

exhibit the properties displayed by CNTs on the nanoscale

is the key to utilizing CNTs in practical applications, such

as chemical sensors, field emission materials, catalyst

supports, capacitive deionization (CDI) and energy storage

devices [5–11]. For this purpose, a significant amount of

research has been performed on the assembling of CNTs

into 3-dimensional (3D) macrostructures that exhibit high

surface areas, controlled porosities, and pore volumes

tunable at the micrometer scale [12]. In order to assemble

CNTs into such macrostructures, several approaches have

been developed. These include using chemical vapor

deposition (CVD), evaporation-induced self-assembly, the

Langmuir–Blodgett technique, and a solution-casting pro-

cess [12–19]. However, most of the CNT assemblies

obtained using these techniques are, in fact, thin films with

a 2-dimensional (2D) architecture. In order to be used

successfully in practical applications and devices at the

macroscopic scale, CNTs must be assembled into truly 3D

macrostructures with controlled structural properties.

The ice-templating (or freeze-casting) method, which

employs an ice-segregation-induced self-assembly (ISISA)

Electronic supplementary material The online version of thisarticle (doi:10.1007/s10934-013-9713-3) contains supplementarymaterial, which is available to authorized users.

S.-H. Park � K.-H. Kim � K.-B. Kim (&)

Department of Materials Science and Engineering, Yonsei

University, 134 Shinchon-dong, Seodaemoon-gu,

Seoul 120-749, Republic of Korea

e-mail: kbkim@yonsei.ac.kr

K. C. Roh (&)

Energy Materials Center, Korea Institute of Ceramic

Engineering & Technology, Seoul 153-801, Republic of Korea

e-mail: rkc@kicet.re.kr

123

J Porous Mater

DOI 10.1007/s10934-013-9713-3

process, has attracted considerable attention in the prepa-

ration of organic and inorganic 3D macrostructures, as well

as those based on CNTs [20–30]. Ice templating involves

the use of ice crystals as a template for the 3D macro-

structures, with the ice crystals being formed during the

unidirectional freezing of an aqueous suspension contain-

ing the organic or inorganic solid particles that need to be

assembled into 3D macrostructures. During the unidirec-

tional freezing process, columns or lamellae of ice grow

along the direction of the heat flow, and the particles

present in the frozen suspension are entrapped between the

formed ice structures. The removal of the ice template by

freeze drying results in the formation of a 3D macro-

structure that is composed of the particles in the suspension

and is a replica of the columnar or lamellar ice structure

[31]. During the ice-templating process, the pore structure

is significantly affected by the growth of the ice crystals, as

it is considered to be governed by the synthesis conditions

such as the initial composition of the suspension, as well as

the conditions during the freezing process [23–32]. Thus,

the ice-templating method can be a simple and versatile

bottom-up process for the fabrication of 3D z-axis-oriented

macrostructures with controlled pore sizes and structure.

Because of these advantages, the ice-templating method has

been recently used to fabricate 3D CNT macrostructures [21,

22]. Since the first report on the fabrication of 3D CNTs mac-

rostructures using the ice-templating method, several studies

have investigated the structural properties of the CNT macro-

structures formed using this technique [33–38]. However, the

structural characteristics (pore size, porosity, morphology, and

structural regularity) of the 3D CNT macrostructures prepared

by the ice-templating method have been inferior to those of 3D

ceramic macrostructures formed using the same technique.

Herein, we report on the parameters critical to the control

of the morphology of 3D CNT macrostructures formed

using the ice-templating method. In order to control the

assembling of CNTs into 3D macrostructures, we system-

atically investigated the effects of various process parame-

ters, such as the initial characteristics of the CNT suspension

used and its freezing conditions, on the properties of the

resulting CNT macrostructures. By adjusting the initial

characteristics of the suspension and its freezing conditions,

we could develop the ice-templating method as an efficient

technique for the fabrication of regular 3D CNT macro-

structures with an aligned lamellar or cellular structure.

2 Experimental

2.1 Preparation of the aqueous CNT suspension

First, an aqueous suspension of CNTs was prepared by dis-

persing them in a chitosan/acetic acid solution. Commercial

CNTs (multiwalled carbon nanotubes; CM-95; mean outer

diameter of 20–40 nm; 1–10 lm in length; purity [95 %;

produced by chemical vapor deposition; Hanwha Nanotech)

were used in this study. It was essential that the CNT sus-

pension be stable since its stability during the ice-templating

process affected the final morphology and structural regu-

larity of the assembled CNT macrostructures. Thus, to sta-

bilize the aqueous CNT suspension, a hydrophilic functional

group was introduced onto the surfaces of the CNTs using an

acid treatment as had been reported previously [5]. To do so,

the CNTs were treated with nitric acid in a glass beaker at

80 �C for 4 h. The acid-treated CNTs were then washed with

distilled water by filtration and dried in an oven.

To examine the effect of the initial characteristics of the

CNT suspension on the final product formed using the ice-

templating method, the concentration of CNTs and that of

chitosan, which acted as a binder, in the suspension, as well as

the viscosity of the suspension, were controlled. First, the

concentration of the binder in the CNT suspension was varied

while keeping the concentration of CNTs in the suspension

constant at 4 wt%. Chitosan powder (medium molecular

mass; viscosity: 200–800 cP, 1 wt% in 1 % acetic acid,

Brookfield, Aldrich) was dissolved in 0.2 M acetic acid in

concentrations ranging from 1 to 4 wt%. The acid-treated

CNTs were then dispersed in the chitosan/acetic acid solution

in concentrations of 1–4 wt% and ultrasonicated for 40 min.

For comparison, a CNT-free suspension of 4 wt% chitosan

was also prepared.

To confirm the effect of the CNT concentration on the

final morphology of the 3D macrostructures, CNT sus-

pensions with different CNT concentrations (1, 4, and 8

wt%) were prepared by the ultrasonication method with the

concentration of chitosan (medium molecular mass) in the

suspension being fixed 1 wt%.

To investigate the effect of the viscosity of the suspension on

the final morphology, three types of chitosan powders with

different molecular masses (low molecular mass: 50–190 kDa;

medium molecular mass: 190–310 kDa, high molecular mass:

310–375 kDa; all purchased from Aldrich) were used. The

viscosities of the suspensions formed using the low-molecular-

mass, medium-molecular-mass, and high-molecular-mass

chitosan powders were 20–200, 200–800, and 800–2,000 cP,

respectively, as measured using the Brookfield method (1 wt%

chitosan powder in 1 % acetic acid). In order to ensure that the

effect of viscosity of the CNT suspension could be observed

readily, the concentration of CNTs in the suspensions was fixed

at 4 wt% and that of the different chitosan powders at 1 wt%.

2.2 Fabrication of 3D CNT macrostructures by ice

templating

The second step was to fabricate 3D CNT macrostructures

from the various aqueous suspensions of CNTs using the

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123

ice-templating method. Figure 1 shows a schematic dia-

gram of the ice-templating system used. It consisted of a

mold for holding the CNT suspension, a digital stage

controller (SGSP 20-85, Sigma Koki Corp.), and a cryo-

genic Dewar flask. In order to promote unidirectional

solidification, we used a cylindrical mold whose curved

surface was made of Teflon (PTFE) and bottom of copper

[inner diameter (D) of 5 mm and height (H) of 10 mm].

The thermal conductivities of Teflon and copper are 0.25

and 401 W/(m K), respectively. To investigate the effect of

the thermal conductivity of the mold on the final mor-

phology of the formed macrostructures, a cylindrical mold

made completely from copper was also used.

The mold was attached to the lower part of the stage,

which was controlled by the digital controller. Then, the

prepared aqueous CNT suspension was poured into the

mold at room temperature. Next, the mold containing the

CNT suspension was dipped into the cryogenic Dewar

flask, the temperature of which was maintained at -196 �C

using liquid nitrogen. The rate of dipping was systemati-

cally controlled using the digital stage controller to range

from 20 to 1,000 lm/s. Once the CNT suspension had

solidified completely, the mold was quickly moved and

placed into a freeze dryer. The solidified CNT suspension

was then freeze dried for 1 day to prevent the formed

structure from collapsing owing to capillary forces.

The morphologies of the fabricated CNT macrostruc-

tures were investigated using scanning electron microscopy

(SEM) (Hitachi).

3 Results and discussion

Figure 2 shows the morphology of a 3D CNT macro-

structure fabricated using the ice-templating method. As

can be seen from the photographic image in Fig. 2a, the 3D

CNT macrostructure was cylindrical and conformed to the

shape of the mold used. The diameter and height of the

obtained macrostructure were 5 and 10 mm, respectively.

However, the size and the shape of the final product were

not limited and could be controlled by using a different-

shaped mold and by altering the volume of suspension

immersed and frozen.

SEM images of the macrostructure (Fig. 2b) confirmed

that the ordered CNT macrostructure comprised large

domains with sizes on the order of 1,000 lm. An SEM

image of the vertical cross-section of the 3D macrostruc-

ture (Fig. 2c) also showed that the macrostructure was well

ordered along the z-axis and contained features with sizes

around 1,000 lm.

As noted previously, in order to promote unidirectional

solidification, the curved surface and bottom of the mold used

were fabricated from Teflon and copper, respectively, which

have different thermal conductivities. To investigate the

effect of the thermal conductivity of the mold on the final

morphology of the macrostructures, 3D CNT macrostruc-

tures were also fabricated using a cylindrical mold fabricated

completely from copper. Figure 2d shows an SEM image of

one such 3D CNT macrostructure. It was observed that, in

contrast to the macrostructures prepared using the Teflon

curved surface/copper bottom mold, the macrostructures

fabricated using the mold with a copper curved surface and

bottom was composed of smaller-sized, randomly oriented

domains. This was owing to the difference in the direction of

the heat flow during the solidification of the CNT suspension

[31]. When using the copper curved surface/copper bottom

mold, the heat flow was distributed and not aligned along a

single direction; this might have disrupted the unidirectional

solidification process responsible for the fabrication of the

aligned, regular CNT macrostructures. In addition, it is pos-

sible that it also led to the formation of the randomly oriented,

smaller-sized structural domains (see Fig. S1 in the Sup-

porting Information). Figure 2e shows a typical SEM image

of the wall of a CNT macrostructure. It can be seen that the

wall of the macrostructure consists of a well-entangled web

of CNTs. Figure 2f shows that the thickness of the wall is

around *1 lm.

Several studies have been published on the fabrication

of CNT macrostructures using methods such as CVD, the

Langmuir–Blodgett technique, and solution-casting pro-

cesses that use silica balls as templates. However, almost

all of these macrostructures were thin films that were

composed of regularly patterned cavities [12–18]. On the

other hand, the CNT macrostructures prepared in this study

Fig. 1 Schematic diagram of the ice-templating system. The ice-

templating system consisted of a mold for holding the CNT

suspension, a digital stage controller (SGSP 20-85, Sigma Koki

Corp.), and a cryogenic Dewar flask that contained liquid nitrogen

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123

using the ice-templating method were truly 3D and were

oriented along the z-axis, as can be seen in Fig. 2.

It is known that the stability of the CNT suspension is

critical to the fabrication of well-aligned CNT macro-

structures. Several studies have reported the fabrication of

CNT macrostructures from bare CNTs using only poly-

meric additives such as chitosan or carboxymethyl cellu-

lose (CMC); these polymeric additives can act not only as

binders but also as dispersion agents for the solubilization

of CNTs, which are hydrophobic, in aqueous solutions

[21, 22]. However, in this study, as described in the

experimental section, the surfaces of the CNTs were

functionalized via an acid treatment to further improve the

degree of dispersion of the CNTs.

Figure 3 shows SEM images of a CNT macrostructure

prepared from bare CNTs and of one prepared from the

acid-treated CNTs. (For both samples, the CNT suspension

consisted of 1 wt% chitosan as binder and 4 wt% CNTs,

with the rate of dipping being constant at 200 lm/s) The

macrostructure prepared from the bare CNTs had a porous

yet disordered morphology (see Fig. 3a) and was similar to

that of previously reported macrostructures prepared from

untreated, bare CNTs [34, 35, 38]. When the acid-treated

CNTs were used, a well-aligned, lamellar macrostructure

Fig. 2 Typical 3D CNT macrostructures prepared using the ice-

templating method; a Photographic image and SEM images of b a

cross-section and c a vertical section across the z-axis of a

macrostructure fabricated using the mold with a Teflon curved

surface and copper bottom, d a cross-section of a macrostructure

prepared using the mold with of a copper curved surface and a copper

bottom, e typical SEM image of the wall of the CNT macrostructure

in figure d and f high-magnification image of the wall of the CNT

macrostructure in figure (d) (The CNT suspension contained 1 wt%

chitosan as a binder and 4 wt% acid-treated CNTs. The rate of

dipping of the mold was fixed at 200 lm/s)

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could be fabricated using the ice-templating method, as

shown in Fig. 3b. On comparing the two CNT macro-

structures, one could see that the use of a stabilized CNT

suspension increased the structural regularity of the

resulting macrostructure [21, 22, 34, 35, 38]. This con-

firmed that the stability of the CNT suspension used was

one of the most important parameters in the fabrication of

aligned, regular CNT macrostructures.

During the ice-templating process, it is likely that the

concentrations of the binder and CNTs in the suspension

have an effect on the final morphology of the CNT mac-

rostructures. In order to be able to fabricate ordered 3D

CNT macrostructures, we controlled the concentration of

chitosan in the CNT suspension. The concentration of

chitosan was varied from 1 to 4 wt% while that of the

CNTs was kept constant at 4 wt% in the suspension. For

comparison, a CNT-free suspension containing 4 wt% of

chitosan was also prepared.

Figure 4 shows the SEM images of various 3D CNT

macrostructures formed using suspensions with different

concentrations of the binder, with the rest of the freezing

conditions being the same. (For all the samples, the rate of

dipping of the mold was constant at 200 lm/s.) When the

concentrations of chitosan and CNTs were 1 and 4 wt%,

respectively, the macrostructure fabricated had an ordered

lamellar structure. It could be seen clearly that the walls of

macrostructure were formed of arrays of well-ordered

lamellae with a gap of approximately 10 lm between the

walls.

When the concentration of the binder in the suspension

was 2 wt% with the concentration of the CNTs still at 4

wt%, the morphology of the resulting macrostructures was

less ordered and the macrostructures consisted of non-

aligned lamellae. For a binder concentration of 4 wt% and

CNT concentration of 4 wt%, the macrostructures formed

consisted of randomly oriented pores (Fig. 4c). These

macrostructures were similar in morphology to the ones

prepared using the CNT-free suspension containing 4 wt%

chitosan, as shown in Fig. 4d. Additionally, these two types

of macrostructures were also similar to previously reported

macrostructures prepared from pure chitosan suspensions

(i.e., those that did not did not contain any other compo-

nents) using the ice-templating method [32, 33]. This

indicated that the morphology of the macrostructures

formed using the ice-templating method were affected the

most by the structure of chitosan, which changed with an

increase in its concentration in the CNT suspension.

As a result, it could be assumed that the properties of the

CNT suspension significantly affected the fabrication of

well-ordered 3D macrostructures via the ice-templating

method. Finally, to investigate the effect of the concen-

tration of CNTs in the suspension, suspensions with vary-

ing CNT concentrations (1, 4, and 8 wt%, with the

concentration of chitosan fixed at 1 wt%) were used to

form macrostructures. For a CNT concentration of 1 wt%,

the formed macrostructure was very soft and mechanically

weak owing to its high porosity (data not shown). On the

other hand, the suspension containing 8 wt% CNT was too

viscous and unstable, owing to the high concentration of

CNTs in the suspension. We also observed that, in this

case, CNTs precipitated to the bottom of the container even

though the suspension, which contained acid-treated CNTs,

had been ultrasonicated for 1 h. Therefore, when system-

atically examining the effect of the viscosity and cooling

rate of the CNT suspension, the concentrations of CNTs

and chitosan in the suspension were fixed at 4 and 1 wt%,

respectively.

Since pores were formed in the macrostructures owing

to the use of ice crystals as a template, it was assumed that

the size of the pores could be adjusted by controlling the

growth of the ice crystals (i.e., the freezing conditions). To

do so, the rate of dipping of the CNT suspension-contain-

ing mold into liquid nitrogen was varied. This, in turn,

varied the rate at which the CNT suspension was cooled.

(In order to be able to observe the effects of the rate

of cooling of the CNT suspension with ease, the

Fig. 3 SEM images of a CNT macrostructure prepared from a pristine CNTs and b acid-treated CNTs. (The CNT suspension used for the two

samples contained 1 wt% chitosan as a binder and 4 wt% CNTs. The rate of dipping of the mold was fixed at 200 lm/s)

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concentrations of CNTs and chitosan in the suspension

were fixed at 4 and 1 wt%, respectively, in keeping with the

conditions for the formation of lamellar macrostructures.)

Figure 5 shows the morphologies of the various 3D

CNT macrostructures fabricated by varying the dipping

rate from 100 to 1,000 lm/s. At a dipping rate of 100 lm/s,

the macrostructure formed was lamellar and had an average

gap of 25 lm between the walls (Fig. 5a). When the dip-

ping rate was increased to 200 lm/s, the average gap

decreased from 25 to 15 lm (Fig. 5c, d). On further

increasing the dipping rate to 500 lm/s, the average gap

decreased to 10 lm. Furthermore, side branches were

observed between the walls of the lamellae of this mac-

rostructure. As mentioned above, the pores of the macro-

structures fabricated using the ice-templating method are

replicas of the ice crystals formed during the process, and

the freezing conditions strongly affect the final morphology

of the macrostructures. At higher cooling rates, superco-

oling takes place in the areas preceding the freezing front;

this can result in a reduction in the size of the ice crystals

formed [27, 31, 39]. As a result, the CNT macrostructure

thus produced would have smaller pores. On the other

hand, under a slow cooling regime, the ice crystals would

be noticeably larger in size, resulting in an increase in pore

size. In the case of the highest dipping rate (1,000 lm/s), a

sparsely ordered porous macrostructure was fabricated, as

can be seen from Fig. 5g. This can be attributed to there not

being enough time for the CNTs to be ejected in front of

the ice crystals owing to the high cooling rate. As a result,

the CNTs were not separated between the ice crystals but

trapped within them, thus leading to the formation of the

sparsely ordered macrostructure.

Since the first report on the fabrication of 3D macro-

structures using the ice-templating method, by Gutierrez

et al., several researchers have further developed the

technique [21, 22, 33–39]. However, in contrast to ceramic

macrostructures prepared using the ice-templating method,

improvements in the morphologies of the CNT macro-

structures that can be fabricated using this technique have

been limited [27, 31].

Mukai et al. [25] and Hortiguela et al. [40] had reported

that the morphologies of the resulting macrostructures were

significantly affected by the mobility of the particles in the

suspension used. They had suggested that the morphology

of the fabricated macrostructures could be controlled by

varying the gelation time of the suspension. The final

structure of the macrostructures varied significantly with

changes in the gelation time of the suspension, with the

structure changing from flat, fiber-like to lamellar and

microhoneycomb-like, with the rest of the freezing condi-

tions remaining constant [25]. Although the exact mecha-

nism responsible for this change in structure remains

Fig. 4 Morphologies of the 3D CNT macrostructure fabricated using

suspensions containing CNTs and chitosan in different concentra-

tions: a 4 wt% CNTs and 1 wt% chitosan, b 4 wt% CNT and 2 wt%

chitosan, c 4 wt% CNTs and 4 wt% chitosan, and d 4 wt% chitosan

with no CNTs being present. (Rate of dipping for all samples:

200 lm/s.)

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unclear, it is likely that the mobility of the particles in the

suspension affects and governs the shape of the resulting

macrostructures [40].

On the basis of this previously reported result, we

adjusted the viscosity of the CNT suspension by using

chitosan samples of different molecular masses. By doing

so, we were able to control the mobility of the CNTs in the

suspension. Figure 6 shows the morphologies of the vari-

ous CNT macrostructures formed using chitosan samples

of different molecular masses while the concentrations of

Fig. 5 Morphologies of the 3D CNT macrostructures formed using different dipping rates: a and b 100 lm/s; c and d 200 lm/s; e and f 500 lm/s;

and g and h 1,000 lm/s (The CNT suspension contained 1 wt% chitosan as a binder and 4 wt% acid-treated CNTs.)

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the CNTs and chitosan were 4 and 1 wt%, respectively.

With an increase in the molecular mass of chitosan, the

morphology of the formed CNT macrostructures changed

dramatically: they went from having a disordered, lamellar

structure to having a well-aligned, lamellar one, and finally

to one that was cellular (or honeycomb-type). Figure 6c

shows an ordered, cellular CNT macrostructure fabricated

using a suspension containing high-molecular-weight

chitosan. On comparing this cellular macrostructure with a

lamellar one, one could see clear differences in shapes and

morphologies of the two. The walls of the cellular mac-

rostructure were elliptical or semicylindrical and inter-

connected. The average gap between the walls was 20 lm

and the wall thickness was 1 lm. Mukai et al. had reported

that lamellar structures were formed when suspensions

containing highly mobile particles were used and that

cellular structures were formed when suspensions con-

taining particles with low mobilities were used. The results

of this study corresponded well with those reported by

Mukai et al. [25].

The structural characteristics of 3D macrostructures,

including their porosity, have a significant effect on their

use in various applications. The full potential of 3D mac-

rostructures will only be achieved when the characteristics

of their pores can be systematically controlled. By adjust-

ing the initial characteristics of the CNT suspension used

and its freezing conditions, we could develop the ice-

templating method as an efficient technique for the fabri-

cation of regular 3D CNT macrostructures with aligned

lamellar or cellular structures. Thus, the ice-templating

method is a simple and versatile bottom-up process and

allows for the fabrication of 3D pore- and structure-con-

trolled macrostructures that are oriented along the z-axis.

4 Conclusions

On the basis of an experimental study of the ice-templating

method for the fabrication of 3D CNT macrostructures

using suspensions of different initial characteristics, which

were frozen under different conditions, the following

conclusions could be made:

1. 3D CNT macrostructures with domains with sizes in

the range of 1,000 lm can be prepared by the ice-

templating method. During the ice-templating process,

the thermal conductivity of the mold and the stability

of the CNT suspension have a significant effect on the

fabrication of well-aligned 3D CNT macrostructures.

2. The structural features of ordered 3D lamellar micro-

structures fabricated using the ice-templating method

can be controlled by the changing the rate of dipping

Fig. 6 Morphologies of the 3D CNT macrostructures formed using

chitosan samples of different molecular masses: a low-molecular-

mass chitosan, b medium-molecular-mass chitosan, c high-molecular-

mass chitosan, and d magnified image of the macrostructure in

(c) (The CNT suspension contained 1 wt% chitosan having different

molecular masses and 4 wt% acid-treated CNTs. The rate of dipping

of the mold was fixed at 200 lm/s)

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of the CNT suspension into the liquid nitrogen bath,

since this changes the cooling rate of the suspension.

3. By increasing the molecular mass of the chitosan

sample used in the CNT suspension, the morphology

of the resulting CNT macrostructures can be changed

dramatically: it can be changed from being disordered

and lamellar to being well-aligned and lamellar and,

finally, to cellular (or honeycomb-like). The ice-

templating method is thus a simple and versatile

bottom-up process and allows for the fabrication of 3D

pore- and structure-controlled macrostructures ori-

ented along the z-axis.

Acknowledgments This work was supported by the Energy Effi-

ciency & Resources of the Korea Institute of Energy Technology

Evaluation and Planning (KETEP) grant funded by the Korea gov-

ernment from the Ministry of Knowledge Economy, Korea (No.

20122010100140 and No. 20122010100090).

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