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This journal is c The Royal Society of Chemistry 2011 Chem.
Commun.
Cite this: DOI: 10.1039/c1cc10707k
Microstructural control and selective C2H5OH sensing
properties
of Zn2SnO4 nanofibers prepared by electrospinningw
Seung-Hoon Choi,a In-Sung Hwang,b Jong-Heun Lee,b Seong-Geun Oha
and Il-Doo Kim*c
Received 6th February 2011, Accepted 30th March 2011
DOI: 10.1039/c1cc10707k
Microstructural evolution of spinel Zn2SnO4 nanofibers was
manipulated via an in situ phase separation process of
inorganic
precursors and a matrix polymer during electrospinning and
calcination. Chemiresistive gas sensors using porous
Zn2SnO4fibers exhibited superior C2H5OH sensing response.
One dimensional (1D) metal oxide nanostructures, such as
wires, tubes, rods and belts, have been explored extensively
for
potential applications in chemical sensors, catalysts,
rechargeable
batteries, photonic devices, and transistors.1 The
electrical,
optical and catalytic properties of these 1D metal oxides
are
strongly correlated with their shape, size and structures.
Among various synthesis methods of 1D metal oxides, electro-
spinning (e-spin), which utilizes target material precursors
and
a matrix polymer,2 is one of the simplest, most
cost-effective,
and versatile techniques for creating ceramic nanofibers.
Various
metal oxide nanofibers such as simple binary oxides3 (i.e.,
ZnO,
TiO2, SnO2, and NiO), complex oxides such as LaNiO3,4 and
two-phase mixtures of NiO/ZnO
5
have been reported. Inaddition, the morphologies of e-spun metal
oxides have been
successfully manipulated by controlling processing
conditions,
leading to different geometries such as nanorods on nano-
fibers, belts, hollow fibers, cable-like multi-core fibers,
and
bicomponent fibers.6 Thus, multiple, superior functions can
be
incorporated into metal oxide fibers to extend their unique
functional features for various applications.
Zinc stannate (Zn2SnO4) has a cubic inverse spinel structure
with a lattice parameter of B8.65 A and is known as a
transparent n-type semiconductor (Eg = 3.6 eV) with high
electron conductivity (B104 S cm1) and fascinating optical
properties.7 Based on zinc stannates high electrical
conductivity
and low absorption coefficient in the visible range, as well
ashigh chemical sensitivity, various Zn2SnO4 nanostructures
such as particles, belts, cones, and wires have been
considered
as potential candidates for ultraviolet photodetectors,8
photocatalysts for decomposition of organic pollutants,9
working electrodes for dye-sensitized solar cells,10 sensors
for
moisture and combustible gases,11 and anode materials for
Li-ion batteries.12 However, to the best of our knowledge,
there have been no reports in the literature on the
fabrication
of inverse spinel Zn2SnO4 nanofibers and in-depth study on
the microstructural evolution of Zn2SnO4 precursor/polymer
composite fibers during the e-spin and calcination process.
Although an earlier study has reported on the fabrication
of ZnO/SnO2 composite fibers via the e-spin technique,13
morphological design of Zn2SnO4 nanofibers via the control
ofin situ phase separation between an inorganic precursor
and
a matrix polymer has not been investigated. Here, we report
on the facile synthesis of porous and dense Zn2SnO4 nano-
fibers and ultra-selective C2H5OH gas sensing
characteristics
against H2 and CO gases.
The formation mechanism of Zn2SnO4 nanofibers with different
morphologies, i.e., porous and dense fibers, is described in
the
ESIw (Experimental section and Fig. S1). The morphologies of
e-spun ceramic fibers are strongly influenced by the
miscibility
between the inorganic precursor and polymer.14 This
indicates
that miscibility control between the precursor and polymer
is
one of the key parameters to control the interior
morphologies
of e-spun metal oxide fibers.
In order to verify this, we prepared Zn2SnO4 fibers using
e-spin of the precursor solution containing zinc acetate
(Zn(OAc)2), tin(IV) acetate (Sn(OAc)4) and two different
kinds
of polymers, polyvinylacetate (PVAc, Mw: 1300000 g mol1)
and polyvinylpyrrolidone (PVP, Mw: 1 300 000 g mol1).
Fig. 1ad present FE-SEM images of Zn2SnO4 fiber mats,
which were prepared using a PVAc based polymer and a
ZnSn precursor solution. As-spun Zn(OAc)2Sn(OAc)
4/PVAc
composite fibers exhibit randomly oriented fibers in the
form
of nonwoven mats with diameters ranging from 380 to 718 nm
and lengths of several hundred micrometres (Fig. 1a).
Calcina-
tion of the composite fibers at 700 1C resulted in the
formation
of polycrystalline Zn2SnO4 fibers due to decomposition of
the PVAc matrix and crystallization of
Zn(OAc)2Sn(OAc)4precursors into inverse spinel Zn2SnO4. Diameters
of the
Zn2SnO4 fibers ranged from 352 to 705 nm, which are similar
values to those of as-spun fibers. The calcined Zn2SnO4
fibers
exhibited a porous surface and lotus-root-like morphologies
(Fig. 1c and d). Highly porous features were clearly
observed
in the scanning TEM (STEM) analysis (Fig. 1e).
Nanocrystallites,
a Department of Chemical Engineering, Hanyang University,Seoul
133-791, Republic of Korea
b Department of Materials Science and Engineering, Korea
University,Anam-Dong, Seongbuk-Gu, Seoul 136-713, Republic of
Korea
c Department of Materials Science and Engineering,Korea Advanced
Institute of Science and Technology,Daejeon 305-701, Republic of
Korea. E-mail: [email protected];Fax: +82-42-359-3310; Tel:
+82-42-350-3329w Electronic supplementary information (ESI)
available. See DOI:10.1039/c1cc10707k
ChemComm Dynamic Article Links
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Chem. Commun. This journal is c The Royal Society of Chemistry
2011
which are smaller than 20 nm, are clearly visible in Fig. 1f.
The
composition profile of Zn2SnO4 fibers revealed that the
Zn/Sn
chemical composition ratio was approximately 2 : 1 (Fig.
1g).
Careful examination of HR-TEM images (Fig. 1h) revealed
cubic inverse spinel structures. Interplanar distances of 3.08 A
,
2.64 A , and 5.02 A were observed with angles of 31.21 and
581
between the respective planes. These planes correspond to
the
(220), (311), and (111) planes of Zn2SnO4. On the contrary,
in
the case of Zn2SnO4 fibers prepared from a PVP polymer and
ZnSn precursor solution, the resultant fibers exhibited a
smooth surface and a dense inner structure composed of fine
particles (Fig. 1i). The X-ray diffraction patterns of both
Zn2SnO4 fibers displayed similar polycrystalline diffraction
peaks of the cubic inverse spinel phase characterized by 2y
peaks = 29.21, 34.41, 35.951, 41.781, 45.71, 51.71, and
55.11,
which represent the (220), (311), (222), (400), (331), (422),
and
(511) planes, respectively (corresponding to PDF 24-1470)
(Fig. 1j).
Fig. 2 shows a representative TGA curve of Zn(OAc)2
Sn(OAc)4/PVAc composite fibers and longitudinal
cross-sectional
images of a focussed ion beam (FIB)-cut single fiber at each
calcination step of 10 min. As-spun Zn(OAc)2Sn(OAc)4/PVAc
composite fibers are composed of an inhomogeneous mixture of
two distinctive phases (Fig. 2a), i.e., a
Zn(OAc)2Sn(OAc)4-rich
domain and a PVAc-rich domain (see ESIw, Fig. S2). Below
300 1C, residual N,N0-dimethylformamide (DMF) and water in
the as-spun fibers are slowly vaporized (region I in Fig. 2).
The
degradation process of the composite fibers exhibits two
main
distinctive regions (II-1 and II-2 regions in Fig. 2). In the
first
step, the composite fibers begin to lose weight at 300 1C and
this
weight loss continues up to 325 1C (region II-1 in Fig. 2).
This
indicates the creation of the thermal decomposition of the
organic
group mostly associated with acetate groups of Zn(OAc)2
Sn(OAc)4 and deacetylation of VAc (vinylacetate) groups in
PVAc to form polyacetylene segments in the backbone.3,13
When the polymer decomposition temperature (300 1C) is
first reached, inner fiber morphologies composed of solid-
state ZnSn precursor domains maintained their initial shape
(Fig. 2b). As the temperature was increased up to 450500 1C,
the unsaturated carbon backbone and organic composites
burned out (region II-2 in Fig. 2) and ZnSn precursors were
crystallized, retaining their highly porous inner morphology
(Fig. 2c). The lotus-root-like morphology remains unchanged
after heat-treatment at 700 1C (region III in Fig. 2 and Fig.
2d).
In contrast, as-spun Zn(OAc)2Sn(OAc)4/PVP composite fibers
exhibited no apparent interface between the precursors and
PVP (middle upper image in Fig. 2), leading to the formation
of dense fibers after calcination at 700 1C (see ESIw, Fig.
S3).
This indicates that Zn(OAc)2Sn(OAc)4/PVP composites have
better miscibility than that of Zn(OAc)2Sn(OAc)4/PVAc
composites. This is due to the fact that PVP binds with
inorganic
salts, as compared to PVAc, because a strong withdrawing
pyrrolidone group in PVP facilitates the association with
salt.15
In order to examine the gas response performance, we
measured the response of sensor prototypes comprising net-
works of porous and dense Zn2SnO4 nanofibers, respectively
(see Fig. 3ac). Both the porous and dense Zn2SnO4 fibers
showed typical n-type gas sensing behaviors, i.e., a
resistance
decrease by reducing gases (100 ppm CO, H2, and C2H5OH).
Ra/Rg ratios were used to evaluate the gas responses to
reducing
gases.16 Here, a maximum sensor response to 100 ppm of
C2H5OH was achieved at an operating temperature of 450 1C
(Fig. 3dg). The gas sensing transient plots of both Zn2SnO4
Fig. 1 Morphologies and crystal structures of Zn2SnO4 fibers
using
different polymer matrixes: (a) SEM image of as-spun ZnSn
precursor/
PVAc composite fibers; (b) SEM image of Zn2SnO4 fibers calcined
at
700 1C (PVAc matrix); (c) magnified SEM image of (b); (d)
cross
sectional image of (c); (e) scanning TEM (STEM) image of (c);
(f) and
(g) TEM image and EDS elemental mapping of Zn, Sn; (h)
HR-TEM
image and lattice fringe of selected area (the values in
parentheses
correspond to the theoretical results); (i) SEM image of Zn2SnO4
fibers
calcined at 700 1C (reference, PVP matrix); (j) XRD pattern of
Zn2SnO4fibers.
Fig. 2 TGA curve of Zn(OAc)2Sn(OAc)4/PVAc composite fibers;
inset shows longitudinal cross-sectional FESEM images of a
single
fiber taken from each calcination step using FIB milling.
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This journal is c The Royal Society of Chemistry 2011 Chem.
Commun.
fibers at 1100 ppm of C2H5OH exhibited stable response and
recovery characteristics (Fig. 3d and e). The gas response
to
100 ppm of C2H5OH of PVAc based porous Zn2SnO4 nano-
fibers (Ra/Rg: 300) was 3.75 times higher than that of PVP
based
dense Zn2SnO4 nanofibers (Ra/Rg: 80) (Fig. 3f).
Considering the different microstructures of the Zn2SnO4fibers,
the enhanced C2H5OH sensing characteristics of porous
Zn2SnO4 fibers can be attributed to their highly porous
struc-ture with a higher surface area (29.02 m2 g1) and larger
accessible
pore volume than that (11.06 m2 g1) of dense Zn2SnO4nanofibers,
which facilitate fast gas transport and effective
surface reaction (see ESIw, Fig. S4). In addition, the
cross-
responses to 100 ppm of CO and H2 were negligible (Fig. 3g).
This demonstrated the potential for selective and sensitive
detec-
tion of C2H5OH at 450 1C using porous Zn2SnO4 nanofiber
networks.
In summary, the morphology of Zn2SnO4 fibers was strongly
affected by the miscibility between Zn(OAc)2, Sn(OAc)2 and
the matrix polymer. We applied porous and dense Zn2SnO4fibers to
semiconducting gas sensors. Remarkably high
selectivity for C2H5OH against CO and H2 gases was observed
in both porous and dense Zn2SnO4 sensors. In particular, the
porous Zn2SnO4 sensor exhibited approximately 4-fold higher
C2H5OH sensitivity compared to the dense Zn2SnO4 sensor,
leading to a new player for application in volatile organic
compound sensors. The proposed synthetic method is simple
and versatile, providing fascinating opportunities to
control
the morphology of various complex metal-oxide nanofibers,
particularly optimized for applications in gas sensors.
This work was supported by a grant from the cooperative
R&D Program (B551179-10-01-00) funded by the Korea
Research Council Industrial Science and Technology, Republic
of Korea. The work of J.-H. Lee was supported by the KOSEF
NRL Program (R0A-2008-000-20032-0).
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Fig. 3 (a) Confocal laser micrograph of porous Zn2SnO4
fibers
coated on the Al2O3 substrate with two Au electrodes; SEM
image
of (b) porous Zn2SnO4 fibers in a selected area of (a); (b1)
enlarged
image of (b); (c) dense Zn2SnO4 fibers on the Al2O3 substrate
with two
Au electrodes; (c1) enlarged image of (c); (d) and (e) dynamic C
2H5OHsensing transient of porous and dense Zn2SnO4 fibers as a
function of
C2H5OH concentration at 450 1C; (f) gas responses (Ra/Rg, Ra:
resistance
in air and Rg: resistance in gas) from 1 ppm to 100 ppm of
C2H5OH at
450 1C; (g) gas responses to 100 ppm of CO, H2, and C2H5OH
operated
at 450 1C.
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