Controlled synthesis of porous Co3O4 nanofibers by spiral electrospinning and their application for formaldehyde oxidation

Article abstract of DOI:10.1039/c6ra21172k

Nanofiber mats have been widely used in various fields owing to their high porosity, high specific area and three-dimensional architecture. Porous cobaltosic oxide (Co₃O₄) nanofiber mats were mass-produced by spiral electrospinning and controlled calcination, after which the three-dimensional scaffold still existed and consisted of well-twisted continuous nanofibers. The nanofiber was composed of neat Co₃O₄ nanoparticles with a necklace-like arrangement. The mechanism for the formation of porous Co₃O₄ necklaces was proposed by investigating the structural evolution of the calcined fibers, their elemental composition, crystal structure and the presence or absence of different functional groups. Moreover, the porous Co₃O₄ nanofiber mats exhibited high catalytic activity (98 °C, 100% conversion) and catalytic stability (160 h, nearly 100% conversion) for the oxidation of formaldehyde, which was mostly attributed to the increases in the porosity and the specific area.

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PAPER
Controlled synthesis of porous Co3O4 nanofibers by spiral-
electrospinning and their application for formaldehyde oxidation
Yuxiao Wua, Ming Maa, Bing zhanga, Yunhua Gaoa, Weipeng Lu*a, Yanchuan Guo*a
Received 00th January 2016,
Accepted 00th January 2016
Nanofiber mats have been widely used in various fields owing to their high porosity, high specific area and three-
dimensional architecture. Porous cobaltosic oxide (Co3O4) nanofiber mats were mass-produced by spiral-electrospinning
and controlled calcination, in which the three-dimensional scaffold still existed and consisted of well-twisted continuous
nanofibers. The nanofiber was composed of neat Co3O4 nanoparticles with a necklace-like arrangement. The mechanism
for the formation of porous Co3O4 necklaces was proposed by investigating the structural evolution of the calcined fibers,
their elemental composition, crystal structure and the presence or absence of different functional groups. Moreover, the
porous Co3O4 nanofiber mats exhibited high catalytic activity (98 °C, 100 % conversion) and catalytic stability (160 h, nearly
100 % conversion) for the oxidation of formaldehyde, which was mostly attributed to the increases of the porosity and the
DOI: 10.1039/x0xx00000x
www.rsc.org/
specific
area.
to non-uniform nanofibers.22 Therefore, needleless
electrospin-ning setups had attracted much attention over the
past decades. Lu et al. designed a metal rotating cone as the
Introduction
Metal-oxide nanofiber mats have attracted intensive interest
owing to their unique catalytic, optical and physicochemical
properties.1–5 These properties are closely related to their
shape, high special area and high porosity.6-9 Conventional
synthetic approaches that have been applied for the
fabrication of nanofibers, such as drawing, self-assembly,
electrospinning, phase separation and template synthesis.10-14
Among them, electrospinning has attracted more academic
attention owing to its noticeable advantages over these
spinneret to electrospin
a
polyvinylpyrrolidone (PVP)
solution.23 Lomov et al. presented a novel technique using
needle-disk as spinneret to enhance nanofiber throughput and
maintain high quality nanofibre.24 Bhattacharyya et al.
produced poly(vinyl alcohol) nanofibers by the method of free
surface electrospinning from a wire electrode.25 Meanwhile, a
number of researchers have made great efforts to design new
nanostructured fibers with electrospinning. For example,
Wang et al. prepared porous Fe2O3 nanotubes by generic
electrospinning and calcining;26 Li et al. synthesized hollow
ZnO–SnO2 core–shell nanofibers by electrospinning and a
hydrothermal method;27 Mauro et al. obtained SiO2
microsphere nanofibers by combing electrospinning and sol–
gel processes;28 Zhan et al. synthesized magnetic mesoporous
γ–Fe2O3@Ti0.9Si0.1O2 core–shell nanofibers using sol–gel
chemistry combined with coaxial electrospinning.29 The
nanofibers with unique nanostructures preferably met the
demands of the specific application. However, the formation
mechanism of the microstructure needs to be further
investigated to achieve controllable fabrication of nanofibers.
Formaldehyde (HCHO) has been regarded as an indoor air
pollutant owing to its serious and hazardous effects on human
health.30 Various methods have been used to combat
formaldehyde pollution, such as adsorption, photo-catalysis,
plasma technology and catalytic oxidation.31-33 Among those
methods, low-temperature catalytic oxidation has been
regarded as the most promising method, which degrades
formaldehyde into carbon dioxide and water; the choice of
catalytic materials is important. Currently, transition-metal
oxides and composite oxides have been tested for catalytic
techniques,
including
simplicity,
adaptability,
cost
effectiveness and easy fabrication.12-14 Metal-oxide nanofibers
prepared from electrospinning have found many important
applications in various fields such as catalysis, adsorption,
chemical gas sensors and lithium ion batteries.15-20 However,
the inability to mass-production and design neat
nanostructures are still two important factors that limit the
catalytic efficiency and practical application of metal-oxide
nanofibers.
In recent years, many high-throughput configurations have
been designed for the mass-production of nanofibers. Single-
needle electros-pinning, as a standard electrospinning setup, is
inefficient for commercial applications of nanofibers owing to
the low production rate (0.01–0.1 g/h).21 Simply increasing the
nozzle number may improve the production rate of nanofibers,
but a complicated interaction between the nozzles would lead
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oxidation of formaldehyde, but high reaction temperatures parallel tests were carried out to analyze the properties of the
and moderate catalytic efficiency are obstacles to their nanofibers.
application. There are two ways to enhance their catalytic
Characterization
activity: one is to dope with a noble metal to provide sufficient
metal active sites for the oxidation of HCHO, such as Pt/TiO2,
Morphological investigations of the samples were observed by
Ag/MnO2, Au/ZrO2, Au/Fe2O3, Au/Co3O4-CeO2 and Ag/SBA-
an S-4800 scanning electron microscope (SEM, Hitachi, Japan).
15.34-39 These catalysts possess higher HCHO catalytic activity
The functional groups in the samples were characterized by FT-
at low temperature. However, the high cost of supported
IR. X-ray diffraction (XRD) patterns were tested on a Bruker D8
noble metals limits their wide application. The other way is to
Focus X-ray diffractometer using Cu Kα radiation; the data
change the nanostructure or morphology to enhance the
were collected at a scanning rate of 0.1 sec/step over 2θ range
catalytic activity and expose more active sites to the reactants,
from 10 to 80°.The transmission electron microscope (TEM)
such as mesoporous Co–Mn, 3D–Co3O4 and 3D mesoporous
images were recorded on a JEOL JEM-2100 instrument at a
Cr2O3.40-42 Nevertheless, these nanomaterials still face
a
voltage of 200KV. Thermogravimetric analysis (TGA) was
performed on a Rigaku Standard Model thermal analyzer in air
with a heating rate of 10 °C/min. Special surface areas were
calculated by the Brunauer–Emmett–Teller (BET) method and
the average pore sizes were estimated from the adsorption
branch of the isotherm by the Barrett–Joyner–Halenda (BJH)
formula. The elemental analysis was performed on an
Elementar Vario MICRO CUBE (Germany) and ICP-OES, and the
oxygen content was calculated by difference.
common challenge to improve the structure control to
improve the catalytic efficiency.
In this paper, Co3O4 “nano-necklaces” were mass-produced
by spiral electrospinning and controlled calcination. The
productivity was ~20 g/h, which was hundreds of times higher
than that of traditional needle electrospinning. A possible
mechanism for the formation of the controllable Co3O4
nanomaterials was proposed based on the sequential
structural evolution from nanofibers to necklaces. To evaluate
the catalytic properties of these porous Co3O4 necklaces for
the treatment of indoor air pollution, the samples acquired at
different calcining temperatures were applied for catalytic
oxidation of formaldehyde.
Catalytic oxidation of HCHO
The catalytic oxidation of formaldehyde was performed in a
quartz tubular fixed-bed reactor (Φ=10 mm) with 100 mg of
catalyst. HCHO was generated by using a controlled purified air
flow that passed through a HCHO-saturated solution in an
incubator maintained at 0 °C. The total flow rate was 50
mL/min and contained 400 ppm of HCHO. The space velocity
was 30000 mL/(gcat h). The products of the reaction were
analyzed with an on-line Agilent 6890 gas chromatograph
using Propaek-Q as the separation column equipped with an
FID and Ni catalyst converter, which was used to quantitatively
convert the carbon oxides into methane in the presence of
hydrogen before the detector. The HCHO conversion was
calculated based on the yield of CO2 content as follows:
Experimental
Chemical and materials
Cobaltous nitrate hexahydrate (Co(NO3)2•6H2O), ethanol
(>99.7 %) and polyvinylpyrrolidone (PVP, Mw=30,000) were
purchased from Beijing Chemical Reagent Company (Beijing,
China). All the chemicals were analytical grade reagents and
used without further purification.
Preparation of Co3O4 nanofibers
ꢍ
ꢏ
ꢁꢂꢎ ꢄꢐꢑ ꢆꢄꢒ. %
ꢋ
%
ꢌ
=
ꢀꢁꢀꢂꢃꢄꢅꢆꢇꢈꢉꢊꢄꢅ
× ꢓꢔꢔ%
ꢍ
ꢏ
ꢀꢁꢀꢂ ꢆꢄꢒ. %
ꢊꢅ
A PVP solution was prepared by dissolving 30.0 g of PVP
powder in 270 g of ethanol at ambient temperature under
continuous mechanical stirring for 2 h. Then 29.1 g of
Co(NO3)2•6H2O was added to the solution. After complete
dissolution, the solution was poured into the Teflon reservoir
and electrospun for 2 hours. A positive potential of 65 kV was
applied between the spinneret and the grounded collector
with a distance approximately 16 cm. Finally, the fibers were
dried in an oven for 4 h at 60 °C. We obtained 41.2 g of
electrospinning nanofibers and the productivity was 21.6 g/h.
The as-spun nanofibers were annealed from room
temperature to the final temperatures (180, 350, 500, 600,
and 800 °C) in a ceramic crucible with a heating rate of 10
°C/min, and then the tube furnace was cooled to room
temperature. The samples were denoted as S20,
corresponding to the as-spun nanofibers, and S180, S350,
S500, S600 and S800, corresponding to the nanofibers
annealed at 180, 350, 500, 600 and 800 °C, respectively. Six
where [CO2]out and [HCHO]in represent the CO2 concentration
in the products and the HCHO concentration in the flow gas,
respectively.
Results and discussion
TGA and SEM were carried out to investigate the
decomposition process of the Co(NO3)2/PVP composites
nanofibers. As shown in Figs. 1 and 2, the as-spun nanofibers
possessed a continuous fibrous geometry and a smooth
surface without any pore or structural defect. During
annealing, the nanofibers underwent four stages of weight
loss. The first stage occurred from 20 to 180 °C and was
associated with a 16.48 % weight loss, which was attributed to
the loss of physisorbed water. However, the weight loss had
little effect on the fiber morphology; only small projections
appeared on the smooth surface of the nanofibers. The second
stage occurred from 180 to 340 °C and was associated with a
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weight loss of 26.36 %, which was mainly attributed to the the oxidation of residual PVP and carbon from the residue
decomposition of cobalt nitrate and the degradation of PVP. remaining after the decomposition of PVP. The nanofibers
The decomposition of cobalt nitrate led to the formation of were transformed into “necklaces” composed of
Co3O4 nanocrystals, which were aggregated in a confined area interconnected nanosized particles (Fig. 2d). There was almost
to form nanoparticles. The degradation of PVP
no weight loss as the calcining temperature was increased to
600 °C; the only change that occurred during this stage was
the merging and growth of the particles (Fig. 2e). At
temperatures above 600 °C the Co3O4 nanoparticles merged
with adjacent particles, and the obvious shrinkage along the
nanofibers caused the fracture of the fibers (Fig. 2f).
To further elucidate the changes in the structure of the
nanofibers, the TEM images of S350, S500, and S600 were
analyzed. Fig. 3a (S350) clearly shows the formation of Co3O4
nanoparticles and the porous surface structure, which was
consistent with the SEM analysis. Notably, a layer (3–5 nm)
covered the surface of the particles, which was formed by the
residual PVP and the residues from the decomposition of PVP.
The TEM images of S500 were investigated to better
understand the crystal structure of Co3O4 (Fig. 3b), and the
disappearance of the PVP layer suggested the complete
decomposition of PVP. The high resolution transmission
electron microscopy (HRTEM) image of S500 (inset of Fig. 3b,
Fig. 3d) showed clear lattice fringes of 0.467 nm corresponded
to the crystallographic (111) plane of Co3O4, which indicated
that the S500 sample was structurally uniform and well
crystallized. Because of the high temperature and particle
growth, S600 (Fig. 3c) possessed a filled structure with
negligible pore volume between the nanoparticles.
Fig. 1 TGA curve of electrospun Co(NO3)2/PVP nanofibers.
X-ray diffraction (XRD) was performed to investigate the
structural evolution of the fibers, which proceeded from a
smooth fiber to a nano-necklace. As shown in Fig. 4, there was
almost no diffraction peak in the spectrum of S20 and S180,
which indicated that amorphous Co(NO3)2 was evenly
dispersed throughout the entire cross-section of the
nanofibers. PVP was the major constituent of the sample.
Several small peaks appeared in the spectrum of S350,
suggesting that Co had started to gather. Co3O4 nucleated
gradually and grew preferentially on the (311) plane. As the
temperature increased, Co3O4 nanocrystals (S500, S600)
nucleated gradually and grew preferentially on the (111),
(220), (311), (511) and (400) planes, which was similar to
previous literature reports of Co3O4 that
Fig. 2 SEM images of the Co(NO3)2/PVP nanofibers calcined at
different temperatures (a: S20, b: S180, c: S350, d: S500, e:
S600, f: S800).
resulted in shrinkage of the polymer into a small area, and
these changes caused the formation of extensive micropores
on the fiber surface and the decrease of the fiber diameter
(Fig. 2c). The third stage was from 340 to 500 °C and was
associated with a mass loss of 31.82 %, which was ascribed to
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almost disappeared above 500 °C, which indicated that PVP
started to decompose between 180 and 350 °C and completely
decomposed at 500 °C. The intensity of the band at 1648 cm−1,
attributed to the C=O stretching vibrations of PVP, decreased
and even disappeared as the calcining temperature increased
from 180 to 500 °C, suggesting that the decomposition of PVP
was accompanied by the formation of carbon dioxide. The
adsorption bands at 660 cm−1 and 789 cm−1 assigned to Co–O
was more apparent as the temperature increased, especially
above 350 °C. Combined with the XRD analysis, these results
show the crystallization of Co3O4 was accompanied by the
degradation of PVP.
The elemental analysis of the samples is provided in Table 1 and
the EDS spectrums of S20 and S500 were shown in Fig. 6. For all
the samples, the Co content increased from 18.01 to 73.42 %
as the
Fig. 3 TEM images of (a) S350, (b) S500, (c) S600 and d) HRTEM
image of S500.
Fig. 4 XRD spectra of the as-spun nanofibers, electrospun
Fig. 5 FTIR spectra of the as-spun nanofibers and electrospun
nanofibers calcined at different temperatures.
nanofibers calcined at different temperatures.
crystallized at temperature above 300 °C.43,44 The diffraction
peaks of Co3O4 nanocrystals became sharper and the intensity
was stronger as the temperature increased, indicating that
temperature increased from 20 to 800 °C because of the loss in
weight of the sample while the total mass of Co did not vary. A
large amount of N (12.04 %) and H (4.06 %) was present in the
as-spun nanofibers. It would be impossible for each element to
be entirely derived from only one constituent (for example, if
N was entirely from PVP, the PVP content would be over 100
wt.%); therefore, H2O, Co(NO3)2 and PVP coexisted in the S20
(Fig. 6a). Compared with S20, only the mass of hydrogen and
Co3O4 possessed
a highly crystalline structure. All the
diffraction peaks were well indexed to the spinel type
structure (JCPDS No. 65-3103).
The change in composition and nanostructural evolution of
the samples could also be confirmed by FT-IR. As shown in Fig. oxygen decreased in S180, which confirmed that only water
5, the characteristic peak at approximately 3433 cm−1 were
evaporated below 180 °C. As the temperature increased, the
content of C,
disappeared at higher temperatures (500–800 °C). Finally, only
Co and O remained at a Co/O molar ratio of approximately 3:4
(Fig. 6b). These findings indicate that the decomposition of
Co(NO3)2 was accompanied by the degradation of PVP.
H and N decreased sharply, and even
assigned to the –OH group and H–O–H stretching vibrations of
the water molecules. The peak was more obvious in S20 and
S180 owing to the presence of combined water molecules.
There was no distinct difference between the spectra of S20
and S180, which suggested that the decomposition of PVP did
not occur at a temperature below 180 °C. The adsorption
peaks at 2927 cm−1 and 2956 cm−1 were assigned to the
stretching vibrations of the C–H band of PVP, and the peaks
between 1250 cm−1 and 1500 cm−1 originated from the
bending vibration of C–H and C–N. The peaks resulting from
PVP tended to decrease as the temperature increased, and
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Fig. 6 EDS spectrums of (a) S20 and (b) S500
Table 1 Ultimate analysis of the as-spun nanofibers and electrospun nanofibers calcined at different temperatures
Ultimate analysis (%)
S20
S180
S350
S500
S600
S800
Carbon
Hydrogen
Oxygen
Nitrogen
Cobalt
17.95
4.06
21.49
2.69
32.06
3.58
47.93
12.04
18.01
40.06
14.21
21.56
19.90
6.20
26.58
73.42
26.58
73.42
26.58
73.42
38.26
Table 2 Specific area distribution, pore volume distribution and pore diameter distribution of the as-spun nanofbers and
electrospun nanofibers calcined at different temperatures
Materials
S20
S180
S350
S500
S600
S800
Specific area (m2/g)
Pore volume (cc/g)
Pore diameter (nm)
72.16
0.035
6.52
93.35
0.057
5.91
306.77
0.344
4.83
561.32
0.636
4.27
450.99
0.511
4.54
406.34
0.338
4.89
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nanofibers. With the increase of calcining temperature, the
surface area and pore the volumes first increased owing to the
formation of mesoporous channels, and then decreased
because of the particle growth and destruction of the
mesopores at higher temperature (600, 800 °C). Therefore,
S500 had the largest surface area and pore volume, which
indicated that the material should exhibit higher catalytic
activity owing to the increase of catalytic sites. This was
confirmed by the subsequent formaldehyde oxidation
reactions. Moreover, the pore size distribution of the samples
was 4.27–6.52 nm, suggesting that the calcining temperature
had an effect on the pore diameter.
The catalytic activities of the samples were evaluated for the
oxidation of HCHO, as shown in Fig. 8a. At temperatures below
70 °C all the HCHO conversions were quite low (<20.1 %). At
higher temperatures, the S20 and S180 displayed the lowest
catalytic activity, whereby the HCHO conversion was less than
28.69 % at 200 °C. The S500 exhibited the best catalytic
oxidation activity, entirely converting HCHO at 98 °C. For
catalytic mechanism of HCHO oxidation, the adsorption of
substrate was the first step. The HCHO adsorbed on the Co3O4
surface and the Co3+ in Co3O4 was reduced to Co2+ with the
formation of ·CHO surface species. With nucleophilic attack of
surface oxygen species, the ·CHO species were oxidized to
generate HCOO−species and further oxidized to form HCO·3-
species. Then the H2CO3 was generated from HCO·3- species by
Fig. 7 (a) N2 adsorption–desorption isotherms and (b) pore
diameter distribution of the as-spun nanofibers and
electrospun nanofibers calcined at different temperatures.
To characterize these porous nanofibers further, the N2
adsorption–desorption isotherms and the pore diameter
distribution of all the samples are shown in Fig. 7, and a
summary of the specific surface area, pore volume and
average pore diameter data is shown in Table 2. All the
samples exhibited the characteristic type IV isotherms with H3-
type hysteresis loops, indicating the presence of uniform
micro-porous structure. The specific areas of the S20, S180,
S350, S500, S600 and S800 nanofibers were 72.16, 93.35,
306.77, 561.32, 450.99 and 406.34 m2/g, respectively, and the
corresponding pore volumes were 0.035, 0.057, 0.344, 0.636,
0.511 and 0.338 cm3/g. It is evident that the as-spun
nanofibers had a lower surface area and pore volume, which
suggested that mesoporous channels did not exist in the S20
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was investigated. The results showed that the formation of a
mesoporous structure resulted from the migration and
decomposition of the components, and could be controlled by
adjusting the calcining conditions. In addition, the porous
“necklace” was used to catalyze the oxidation of formaldehyde
and displayed superior catalytic performance at 98 °C. This
work provides a practical method for the mass-production of
mesoporous nanofibers, which are good candidates as catalyst
in practical application.
Acknowledgements
The authors acknowledge the financial support of programs of
International Science and Technology Corporation, China
(Project No. 2013DFA50920) and Science and Technology of
Beijing, China (Project No. Z131100005213007) and State
Natural Sciences Fund, China (Project No. 21506236,
51372276).
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