Versatile preparation of monodisperse poly(furfuryl alcohol) and carbon hollow spheres in a simple microfluidic device

Article abstract of DOI:10.1039/c003161e

Monodisperse PFA and carbon hollow microspheres have been prepared without templates in a single synthesis process by a microfluidic methodology.

Full text of DOI:10.1039/c003161e

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Versatile preparation of monodisperse poly(furfuryl alcohol) and carbon
hollow spheres in a simple microfluidic devicew
Yichang Pan, Minhua Ju, Chongqing Wang, Lixiong Zhang* and Nanping Xu
Received 15th February 2010, Accepted 23rd March 2010
First published as an Advance Article on the web 9th April 2010
DOI: 10.1039/c003161e
Monodisperse PFA and carbon hollow microspheres have been
prepared without templates in a single synthesis process by a
microfluidic methodology.
Hollow microspheres composed of polymeric or inorganic
materials are of great interest because of their potential
application as catalyst supports, carriers for cosmetics, drugs,
protein and DNA delivery, electrodes, sensors, and so on.1
They are prepared by various methods such as emulsion/
micelles/vesicle polymerization,2,3 spray drying,4 template
Fig.
1 (a) Schematic illustration of the microfluidic device to
methods,5 and self-template method.6 Recently,
technique was developed to synthesize uniform-sized hollow
microspheres using microfluidic emulsion technique
a new
fabricate monodisperse PFAHMs. (b) Optical images showing
droplets created in the microfluidic device. (c) Schematic diagram
for formation of PFAHMs and CHMs in the microfluidic device.
a
via interfacial polymerization at the surface of the
microdroplet,7–10 which has provided a facile and scalable
approach to fabricate monodisperse hollow microspheres,
such as nylon,7 biopolymer,8 organosilicon9 and TiO2.10
However, only a single product was prepared each time in
these above-mentioned reports. In addition, control of the wall
thickness of the hollow microspheres was seldom investigated.
Here, we present the preparation of monodisperse
poly(furfuryl alcohol) (PFA) hollow microspheres (PFAHM)
in a microfluidic device assembled by inserting a syringe needle
(100 mm i.d.) perpendicularly into a poly(vinyl chloride) (PVC)
tube (1.2 mm i.d., 1.6 mm o.d.), as shown in Fig. 1a. The
PFAHMs in turn can be transformed to microporous carbon
hollow microspheres by carbonization. Thus, both PFA and
carbon hollow microspheres can be obtained in a single
synthesis process. PFA, which can be easily obtained from
polymerization of FA by acidic catalysis, is widely used in
metal-casting molds, corrosion-resistant materials, negative
photoresists, reverse osmosis desalination membranes, and
as a precursor for the fabrication of different nanostructured
carbons.11 Carbon hollow microspheres (CHM) are known for
their intrinsic properties, such as high strength, high thermal
resistance, low density and bio-compatibility. They are
expected to show high performances when used as catalyst
supports,12 electrodes of fuel cells and lithium-ion batteries,13
electrochemical hydrogen storage materials,14 and adsorbents.15
CHMs are commonly prepared by template methods using
silica particles and polymer latex colloids as templates,16 which
requires uniform sacrificial templates and multistep operations
to obtain monodisperse CHMs. In addition, they can also be
prepared by hydrothermal or solvothermal synthesis17 of
organic materials and pyrolysis of organic gases.18 However,
the particle size distribution of the resultant CHMs is broad,
and high temperature, high pressure and long reaction times
have to be applied. To the best of our knowledge, no report
about easy synthesis of monodisperse PFAHMs and CHMs
without templates in a microfluidic device have been given.
Experiments were carried out at room temperature by
feeding an oil solution containing FA, fatty acid methyl ester
(FAME) and a surfactant (Span 80) and an 8 M sulfuric acid
aqueous solution separately into the PVC tube and the syringe
needle (see ESIw). Aqueous solution microdroplets were thus
formed at the end of the needle by shearing force from the oil
phase. Once a water droplet was formed, FA in the oil phase
would diffuse into the droplet, and consequently start to
polymerize upon catalysis by sulfuric acid (Fig. 1b). Thus, a
layer of PFA would be formed at the surface of the aqueous
droplet, resulting in formation of PFA microcapsules containing
aqueous solution (Fig. 1b and c). After further curing and
drying, we could obtain PFAHMs. The advantages of this
technique include easy control of the particle size and wall
thickness of resultant microspheres by changing the flow rates
of the oil and water solutions and the hydraulic diameter of
the needle, with simple adjustment of hollow or solid
structures by changing the concentration of FA in the oil
solution and the residence time of microdroplets in the PVC
tube, and ready functionalization of the microspheres by
adding suitable materials in the aqueous solution.
Fig. 2a and b show typical optical and SEM micrographs of
PFAHMs prepared at flow rates of the aqueous solution and
oil solution of 0.2 and 20 mL hÀ1, respectively. One can see
that the PFAHMs are of good spherical morphology and
uniform particle size. The particle size distribution of products
illustrated in Fig. 2c shows that the sizes of PFAHMs range
State Key Laboratory of Materials-Oriented Chemical Engineering
and College of Chemistry and Chemical Engineering,
Nanjing University of Technology, Nanjing 210009, P. R. China.
E-mail: lixiongzhang@yahoo.com; Fax: 86 25 8317 2263;
Tel: 86 25 8317 2265
w Electronic supplementary information (ESI) available: Experimental,
SEM images and adsorption isotherms. See DOI: 10.1039/c003161e
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from 243 to 281 mm, the average diameter is 262 mm with a
coefficient of variation (CV) value of 2.7%. The CV value is
smaller than 5%, which is the commonly accepted value for
definition of monodispersity of particles.7 Furthermore, we
did not find any broken or deformed products during the
experiment. The hollow structure of these microspheres with a
wall thickness of ca. 90 mm can be obviously observed from the
SEM image of a purposely broken PFAHM (Fig. 2d).
Fig. 3 (a) A plot of the average diameter of PFAHMs as a function
of the flow rate of oil solution at flow rate of aqueous solutions of
0.2 and 0.5 mL hÀ1. (b) SEM picture of PFAHMs prepared with a
50 mm i.d. glass capillary at flow rates for the water and oil phase of 0.2
and 30 mL hÀ1, respectively.
The particle size of PFAHM can be easily adjusted by
changing the flow rates of the aqueous solution and oil
solution (Fig. 3a). At a fixed flow rate of aqueous solution,
the diameter of the resultant PFAHM decreases almost linearly
with increase in the flow rate of oil solution due to the increase
in shear stress imposed on the droplet. On the other hand, at a
fixed flow rate of oil solution, a lower flow rate of aqueous
solution leads to a decrease in the diameter of the resultant
PFAHM. These results are consistent with those of reported
work in a cross-flow microfluidic device,19 which results from
the increase in the shear stress imposed on the droplet caused
by increase in the flow rate of the continuous phase or a
decrease in the flow rate of the discontinuous phase. Therefore,
we can easily prepare PFAHMs with average diameters in the
range of 234–380 mm and a CV of less than 5% by changing
the flow rates of feeds using a needle with a hydraulic diameter
of 100 mm. However, there is a limitation in reducing the
sphere size by changing the flow rates of two phases. To
further decrease the diameter of PFAHM, we replaced the
needle with a glass capillary with 50 mm i.d. The average size of
the thus prepared PFAHMs is 110 mm (Fig. 3b).
In order to adjust the thickness of PFA layer in the
PFAHM, we conducted a series of experiments by changing
the concentration of FA (cFA) in the oil solution and the
residence time (tR) of the microdroplets in the PVC tube using
fixed flow rates of two phases, as listed in Table 1. One can see
that particle diameters of all products are quite close, indicating
that the diameter (dsphere) of PFAHM is mainly determined by
the diameter of the aqueous microdroplets. In addition,
the average wall thickness (dwall) of PFAHMs increases
with increasing concentration of FA in the oil solution
(Table 1, Fig. S1w). PFAHMs with wall thicknesses ranging
from 40 to 90 mm are prepared when the FA concentration is
adjusted from 3.3 to 9.1 wt%. On the other hand, the average
wall thickness of PFAHMs increases with prolongation
of the residence time of microdroplets in the PVC tube
(Table 1, Fig. S1cw), indicating that more FA diffuses from
the oil solution to the aqueous droplets at longer residence
times. Therefore, we speculated that solid PFA microspheres
would be prepared when the residence time of microdroplets
was long enough. As expected, solid PFA microspheres
were prepared at residence times of 20–40 s with a FA
concentration of 9.1 wt% (Fig. S1dw). However, the PFA
spheres were deformed when the residence times were 2–4 s
(Fig. S1e,f, ESIw), quite possibly resulting from weak mechanical
strength of the spheres because of too thin a PFA shell
thickness. In addition, the curing temperature also has strong
influence on the shape of resultant microspheres. When the
curing temperature is above 75 1C, non-spherical PFAHMs
are obtained (Fig. S2w). This may possibly result from rapid
diffusion of water encapsulated in PFAHMs at high
temperatures.
PFAHMs can be easily transformed to CHMs by
carbonization. The resulting CHMs carbonized at 550 1C in
N2 atmosphere still retain spherical morphology, mono-
disperse size and the hollow structure of the corresponding
PFAHMs (Fig. 4, Fig. S3w). Compared with their precursors,
average diameters and average wall thicknesses of resultant
CHMs for samples 1–4 decrease by about 1/4 and 1/2–2/3,
respectively (Table 1). They all exhibit CV values less than 5%,
indicating monodisperse particle sizes. For sample 5, the PFA
spheres are solid spheres while the corresponding carbon
spheres are hollow (Fig. S1d, S3dw). For sample 6, both the
PFA and the corresponding carbon microspheres are solid
spheres (Fig. S3ew). These results possibly result from different
packing densities of PFA in the PFA spheres at different
residence time. N2 adsorption/desorption isotherms of these
CHMs are of type I (Fig. S4w), typical of microporous
materials. The average pore size of these CHMs is 0.43 nm,
close to the reported pore size of 0.48 nm for PFA-derived
molecular sieve carbons.20 The BET surface areas and micropore
volumes of the CHMs for samples 1–4 are quite close, resulting
from similar compact carbon structures (Fig. S5a,bw).
However, CHMs for samples 5 and 6 exhibit smaller BET
surface areas and micropore volumes than samples 1–4, which
may be due to their interior loose structure (Fig. S5cw). The
CHMs exhibited 10% higher CO2 saturation capacities than
commercial carbon molecular sieves (Takeda 3A).21 Furthermore,
the CO2 saturation capacities on CHMs were higher than CH4
saturation capacities (Fig. S6w), suggesting potential application
Fig. 2 (a) Optical micrograph, (b) SEM pictures, (c) particle size
distribution of PFAHMs and (d) a purposely broken PFAHM
prepared using flow rates for aqueous and oil solutions of 0.2 and
20 mL hÀ1, respectively.
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Table 1 Experimental conditions for synthesis and results of PFAHMs and CHMs in the microfluidic devicea
Poly(furfuryl alcohol) hollow microspheres
Carbon hollow microspheres
cFA (wt%)
tR/s
dsphere/mm
dwall/mm
dsphere/mm
dwall/mm
SA/m2 gÀ1
Vp/cm3 gÀ1
Sample
1
2
3
9.1
5
3.3
9.1
9.1
9.1
8
8
8
6
20
40
262
266
275
275
272
268
90
70
40
63
136
134
194
190
205
208
206
200
32
27
21
30
70
100
600
586
593
566
430
389
0.25
0.24
0.25
0.22
0.17
0.15
4
5b
6c
a
b
Experiments were all conducted at flow rates for aqueous and oil solutions of 0.2 and 20 mL hÀ1, respectively. The PFA spheres are solid
c
spheres whereas the carbon spheres are hollow. Both the PFA and the corresponding carbon microspheres are solid spheres.
in CO2/CH4 separation. Furthermore, all these CHMs show
strong mechanical strength, exhibited by difficulty in cutting
the spheres with a sharp blade.
This work is supported by National Natural Science
Foundation of China (No. 20676059), SRFDP (20093221110002),
‘‘Green–Blue’’ Project of Jiangsu Province and the PhD
Innovation Program Foundation of Nanjing University of
Technology (BSCX200801).
The CHMs can be easily functionalized with magnetic
property by simply adding CoSO4 into the aqueous solution
during preparation of the PFAHMs and reduction with
hydrogen during carbonization. Addition of CoSO4 does not
have influence on the particle size and morphology of resultant
magnetic CHMs (Fig. S7a, S8w). These magnetic CHMs can
be easily attracted by a magnet quickly (Fig. S7bw). Their
magnetic property can be adjusted by regulating the amount
of CoSO4, which can be sensed by the speed of attraction.
These results demonstrate that it is straightforward to
fabricate some functional carbon materials.
Notes and references
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In summary, we have demonstrated that monodisperse PFA
hollow microspheres can be prepared without templates in a
simple microfluidic device. By changing the flow rates of oil
and aqueous solutions, residence time of aqueous micro-
droplets in the fluidic device, FA concentration in oil solution
and curing temperature, the particle size and wall thickness of
PFA hollow microspheres can be easily adjusted. These PFA
microcapsules can be easily transformed to microporous
carbon hollow microspheres exhibiting monodisperse particle
size and strong mechanical strength by carbonization. Thus,
both PFA and carbon hollow microspheres can be obtained in
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Fig. 4 (a) Optical micrograph, (b) SEM images, (c) particle size
distribution of CHMs and (d) a purposely broken CHM carbonized
from corresponding PFAHM (sample 1).
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3734 | Chem. Commun., 2010, 46, 3732–3734