UC Davis
UC Davis Previously Published Works
Title
Nanoporous ultra-high specific surface inorganic fibres
Permalink
https://escholarship.org/uc/item/5rx8106j
Journal
Nanotechnology, 18
ISSN
0957-4484
Authors
Ding, Bin
Kanehata, Masaki
Shiratori, Seimei
Publication Date
2007-08-01
Peer reviewed
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University of California
1
Nano-porous ultra-high specific surface inorganic fibers
Masaki Kanehata
1
, Bin Ding
2,3
and Seimei Shiratori
1,3
1
Faculty of Science and Technology, Keio University, Yokohama 223-8522, Japan
2
Fiber and Polymer Science, University of California, Davis, CA 95616, USA
3
Corresponding authors.
E-mail: bding@ucdavis.edu and shiratori@appi.keio.ac.jp
2
Abstract
Nano-porous inorganic (silica) nanofibers with ultra-high specific surface have been
fabricated by electrospinning the blend solutions of poly(vinyl alcohol) (PVA) and
colloidal silica nanoparticles, followed by selective removal of the PVA component. The
configurations of the composite and inorganic nanofibers were investigated by changing
the average silica particle diameters and the concentrations of colloidal silica particles in
polymer solutions. After the removal of PVA by calcination, the fiber shape of pure silica
particle assembly was maintained. The nano-porous silica fibers were assembled as a
porous membrane with a high surface roughness. From the results of
Brunauer-Emmett-Teller (BET) measurements, the BET surface area of inorganic silica
nanofibrous membranes was increased with the decrease of the particle diameters. The
membrane composed of silica particles with diameter of 15 nm showed the largest BET
surface area of 270.3 m
2
g
-1
and total pore volume of 0.66 cm
3
g
-1
. The physical absorption
of methylene blue dye molecules by nano-porous silica membranes was examined using
UV-vis spectrometer. Additionally, the porous silica membranes modified with
fluoroalkylsilane showed the super-hydrophobicity due to their porous structures.
3
1. Introduction
In recent years, porous materials have been of immense interest because of their potential
for applications in fuel cell membranes, tissue engineering, catalysis, sensors, separations,
electrochemical cells, drug delivery, chemical filtration, etc [1-6]. Porous SiO
2
with
controlled porosity and high specific surface is strongly required as it offers great potential
applications such as sensors, electroluminescence materials, photovoltaic materials, etc
[7-9].
On the other hand, the nanofibers prepared by electrospinning method have been
intensively studied for its unique properties [10] and potential applications such as sensors
[4, 5], filters [6], dye-sensitized solar cells [11, 12], tissue engineering [2], and
super-hydrophobic materials [13, 14]. The process of electrospinning is a special case of
the electrospray process which uses electrostatic fields to form and accelerate a liquid jet
from the tip of a capillary [15].
Many applications of electrospun nanofibers could be greatly improved by increasing the
surface area and porosity of the fibers. Towards that aim, a number of groups have tried to
produce porous nanofibers in a versatile and inexpensive way. As a typical method, the
electrospun porous polymer nanofibers can be obtained by selective dissolution [2, 16-18]
or calcination [19-21] for the removal of one phase from fibers with multi-phase. Recently,
Xia et al [22] reported that, by immersing the collector in a bath of liquid nitrogen, porous
polymer fibers can be fabricated through thermally induced phase separation between the
solvent-rich and solvent-poor regions in the fiber during electrospinning, followed by
removal of solvent in vacuo. Additionally, the fabrication of nano-porous carbon fibers was
reported by the electrospinning two immiscible polymer solutions followed by thermal
4
treatment at elevated temperature in an inert atmosphere [23, 24].
The electrospun inorganic nanofibers containing SiO
2
componenet have been reported
by electrospinning the polymer with sol-gel solution [20, 25], sol-gel solution [26], and
polymer solution blended with colloidal silica particles [27], respectively. However, to our
best knowledge, the SiO
2
nanofibers with ultra-high surface area and porosity are difficult
to find in periodicals and monographs.
In this study, we want to fabricate the highly porous SiO
2
nanofibers with ultra-high
surface area and porosity. The water-soluble poly (vinyl alcohol) (PVA) can be introduced
as template material [28] to blend with colloidal silica nanoparticles for preparing the
composite nanofibers. Inorganic silica nanofibers could be obtained by the calcination of
the composite fibers. The morphology, surface area, fiber pore diameter, and total pore
volume of fibrous samples were studied upon the average diameters of silica nanoparticles
and concentrations of silica in polymer solutions. Moreover, the absorption of methylene
blue (MB) dye by fibrous silica membranes and the hydrophobicity of fluoroalkylsilane
(FAS) modified fibrous silica membranes were also investigated.
2. Experimental
Colloidal silica nanoparticles (STS, Nissan Chemical Industries, Ltd.) with the average
particle diameters of 15, 50, and 100 nm were dispersed in water with a silica concentration
of 20 wt%. The template PVA (M
n
66 000, Wako) was dissolved into water with a PVA
concentration of 10 wt% at 80
o
C under stirring. Then, the electrospinning solutions were
obtained by blending colloidal silica and PVA solutions with a silica/PVA solution weight
ratio of 2/3. To fix the concentration of PVA in all electrospinning solutions, the 10 wt% of
PVA solution was diluted to 6 wt%. The detailed compositions of electrospinning solutions
