PRODUCTION OF CARBON NANOTUBE MICROPARTICLES USING
MICROFLUIDIC DROPLETS IN A NON-EQUILIBRIUM STATE
Sakurako Tomii1, Masahiro Mizuno1, Masumi Yamada1*, Yasuhiro Yamada1, Masahito
Kushida1, and Minoru Seki1
1
Chiba University, JAPAN
ABSTRACT
Here we propose facile processes to prepare carbon nanotube (CNT) microparticles using
microfluidic systems. Aqueous droplets containing CNTs were generated in a continuous phase of watersoluble organic solvent at a microchannel confluence. Water molecules in the droplets were dissolved in
the continuous phase and CNTs were concentrated, resulting in the formation of CNT particles. We
successfully prepared spherical particles from multi-walled CNTs (MWCNTs). In addition, another
process was demonstrated to produce particles from single-walled CNTs (SWCNTs) through the usage of
sacrificial hydrogel matrix.
KEYWORDS: Carbon nanotube, Microparticles, Non-equilibrium droplet, Droplet microfluidics
INTRODUCTION
CNTs are being used as functional materials for catalyst supports, electrodes, and biosensors because
of their high conductivity and specific adsorption properties [1]. Recently, microparticles made of CNTs
have been produced, which can benefit from improved handling properties including dispersion,
concentration, and immobilization because of fabrication of CNT materials on a large spatial scale (to
µm). In addition, the controllability of the pore size between accumulated CNT particles is advantageous
for preparing highly efficient catalysts [2]. To date, several methods for preparing CNT microparticles
have been reported [3], but most of them require complicated multistep processes such as hightemperature treatment because of removal of core particles such as polymer, mesocarbon, and inorganic
materials [4].
We have recently proposed a microfluidic process to produce polymeric microparticles utilizing the
dissolution phenomena of droplets in a non-equilibrium state in microfluidic devices [5]. In this study,
we applied this process to the preparation of CNT microparticles.
EXPERIMENTAL
We first fabricated microfluidic devices to
prepare CNT particles (Fig. 1). PDMS-glass
microfluidic devices were fabricated by using
standard soft lithography and replica molding
techniques (Fig. 2a).
We employed a
microcfluidic device having an orifice structure to
generate monodisperse O/W droplets. The width
of the confluence and other microchannel
segments were 50 and 100/200 µm, respectively,
and the depth was uniform, ~100 µm. MWCNT
suspension and SWCNT suspension as the Figure 1. Schematic diagram showing the microfluiddispersed phases, respectively. The concentration ic process to produce CNT dispersion solvent prevents
of MWCNT was changed from 0.1 to 0.5% and the aggregation. Continuous phase was also added
that of SWCNT was 0.1%. Moreover, Sodium from the midst of the microchannel to prevent the
alginate (NaAlg), gelled with the presence of droplet coalescence.
multivalent cations like Ca2+, was added to the only dispersed phase of SWCNT (Fig. 6a). SWCNTs and
NaAlg were condensed by the dissolution of droplets, and finally, Ca-alginate hydrogel was removed by
chelating Ca2+ using trisodium citrate dihydrate (>99% purity). Isopropyl acetate (>99% purity), propyl
978-0-9798064-8-3/µTAS 2015/$20©15CBMS-0001
1924
19th International Conference on Miniaturized
Systems for Chemistry and Life Sciences
October 25-29, 2015, Gyeongju, KOREA
acetate (>97% purity), and ethyl acetate (>99.9%
purity), the water-soluble organic solvents, were
used as the continuous phases. The dispersed
phases (Inlet 1) and continuous phases (Inlet 2 and
3) were continuously introduced into the
microchannel by using syringe pumps at flow rates
of 1, 50 µL/min, respectively (Fig. 2b). Also,
additional continuous phase was introduced at 500
µm downstream from the confluence, to prevent
the formed droplets from coalescing with each
other, at a flow rate of 400 µL/min (Fig. 2c).
droplets generated at the confluence were
gradually shrunk and finally became CNT
particles. By CNT particles. By collecting in the
water-soluble organic solvents, identical with the
continuous phases, CNT particles were further
shrunk until water in their particles was
dehydrated completely. Finally, morphologies of
CNT particles were observed by using an optical
microscope and a scanning electron microscope
(SEM).
Figure 2. (a) Microchannel design. The channel
width and depth were 200 and 100 µm, respectively.
Flow rates from Inlets 1, 2, 3, and 4 were 400, 1, 50,
and 50 µL/min, respectively. (b, c) Micrographs showing the droplets at the first and second confluences,
respectively.
RESULTS AND DISCUSSION
We first examined if the non-equilibrium O/W
droplets containing CNTs are actually formed in microchannel. As shown in Fig. 2b and c, we observed that
the CNT suspension formed monodisperse O/W droplets (diameter of ~85 µm), which gradually shrunk
during flowing through microchannel, and finally
CNT microparticles are generated because of the Figure 3. SEM images of the MWCNT particles, prevan der Waals interactions between CNTs. When pared using (a) isopropyl acetate and (b) propyl acetate as the polar organic solvent. The average diameSEM image of the MWCNT particles prepared
ters were 8 and 20 µm, respectively.
using isopropyl acetate as polar organic solvent was
observed, spherical particles were obtained with an
average diameter of ~8 µm (Fig, 3a). Interestingly,
concaved particles (Φ = ~20 µm) were formed
when propyl acetate was used as the continuous
phase (Fig. 3b). In addition, the size of the CNT
particles was controllable by changing the initial
CNT concentration from 0.1 to 0.5% (Fig. 4). It
showed that the diameter of CNT particles shrunk in
size as the CNT concentration was depressed.
Next, we attempted to prepare SWCNT
microparticles. Particles were formed in the water- Figure 4. Size distibutions of MWCNT particles presoluble organic solvent when 0.1% SWCNT pared using different CNT concentrations.
suspension was used as the dispersed phase and
ethyl acetate was used as the continuous phase (Fig. 5a), but they were not mechanically strong and the
shape was collapsed after drying(Fig. 5b). We therefore proposed a process using NaAlg as sacrificial
hydrogel matrix for increasing mechanical strength of SWCNT particles (Fig. 6a). As a result, we obtained
non-spherical particles composed of SWCNT and alginate hydrogel, which stably maintained their shape
after drying (Fig. 6b) and even after the removal of alginate hydrogel (Fig. 6c). Thus, we succeeded at
preparing SWCNT particles without using core particles with materials except for CNTs.
1925
CONCLUSION
A microfluidic system was presented for preparing CNT microparticles were composed of MWCNTs
and SWCNTs, respectively. Using this method, it is possible to easily and continuously prepare
microparticles consisting only of CNTs. We expected the presented microparticles are useful for various
applications such as functional materials for fuel cell and biosensor by being supported metallic catalyst or
enzyme.
Figure 5. (a) Optical micrograph and (b) SEM image of the prepared SWCNT particles.
Figure 6. (a) Schematic diagram showing the
production process of SWCNT particles using sacrificial matrix of Ca-alginate hydrogel. (b, c) Obtained
SWCNT particles (b) before and (c) after removing
the alginate hydrogel.
ACKNOWLEDGEMENTS
This study was supported in part by Grants-in-aid for Improvement of Research Environment for
Young Researchers from Japan Science and Technology Agency (JST), and for Scientific Research A
(20241031) from Japan Society for Promotion of Science (JSPS).
REFERENCES
[1] K. Balasubramanian and M. Burghard, Chemically Functionalized Carbon Nanotubes, small, 2, 180192 (2005)
[2] J. H. Choi, et al., Sulfur-impregnated MWCNT microball cathode for Li–S batteries, RSC Adv., 4,
16062 (2014)
[3] K. Nakagawa, et al., A novel spherical carbon, J. Mater. Sci., 44, 221 (2009)
[4] J. Shi, et al., Multiwalled Carbon Nanotube Microspheres from Layer-by-layer assembly and Calcination, J. Phys. Chem. C, 112, 11617-11622 (2008)
[5] T. Ono, et al., One-step synthesis of spherical/nonspherical polymeric microparticles using nonequilibrium microfluidic droplets, RSC Adv., 4, 13557 (2014)
CONTACT
* M. Yamada, tel: +81-43-290-3398; [email protected]
1926