Available online at www.sciencedirect.com
Scripta Materialia 68 (2013) 873–876
www.elsevier.com/locate/scriptamat
Bioinspired synthesis and gas-sensing performance of porous
hierarchical a-Fe2O3/C nanocomposites
Fan Yang,a Huilan Su,a,⇑ Yaqi Zhu,a Jianjun Chen,a Woon Ming Laub,c and Di Zhanga,⇑
a
State Key Lab of Metal Matrix Composites, Shanghai Jiao Tong University, Shanghai 200240, China
b
Chengdu Green Energy and Green Manufacturing Technology R&D Center, Sichuan 610207, China
c
Beijing Computational Science Research Center, Beijing 100084, China
Received 28 November 2012; accepted 8 February 2013
Available online 18 February 2013
Pollen grains of Brassica capestris are chosen as biotemplates to synthesize a-Fe2O3/C nanocomposite gas sensors. Thereinto, a
small amount of carbon remains directly from the biotemplate, acts as a matrix for a-Fe2O3 and helps retaining the porous hierarchical architecture of pollen grains. The macro-/mesopore networks on the scaffolds of the biotemplated composites endow them
with superior gas transportability and large surface areas. By virtue of such structure hierarchy, Fe2O3 nanocrystallites and multicomponent traits, the a-Fe2O3/C composite exhibits high sensitivity to methanol and acetone.
Ó 2013 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.
Keywords: Nanostructure; Nanocomposite; Semiconductor; Sensors; Pollen grains
Metal oxide semiconductor (MOS)-based gas
sensors are attracting increasing interest from researchers concerned with the issue of damage to ecosystems
caused by air pollution. The gas-sensing process mainly
involves the surface reactions between the MOS and target gases [1]. Due to their large specific surface areas and
high surface activity, several nanostructures, such as
nanotubes [2], nanorods [3] and nanosheets [4], have
been synthesized that have desirable gas-sensing properties. Furthermore, these nanoscaled structures can be
assembled into porous hierarchical architectures, which
not only provide a large number of channels for gas diffusion, but also possess significantly larger surface areas
for gas-sensing reactions [5]. MOS-based materials with
various sorts of porous hierarchical structures [6–8] were
fabricated and gave rapid, high responses to target
gases. Consequently, such architecture can endow materials with promising gas-sensing performances.
Nature produces a large number of intricate biological structures. Diatoms [9], egg shell membranes [10] and
butterfly wings [11] have all been used to prepare functional materials for optical use, et al. In particular, pollen grains, the male gametophytes of higher plants,
perform functions including the sensing of the chemicals
⇑ Corresponding
authors. Tel.: +86 21 34202584 (H. Su); e-mail
addresses: [email protected]; [email protected]
on stigmas for fertilization and of alien chemicals for defense [12,13]. The open porous supporting scaffold of a
pollen grain facilitates the diffusion of chemicals in the
grain and provide large surface areas where biosensing
reactions can take place. This sophisticated architecture
developed from evolution is one peculiar kind of porous
hierarchical structure, which makes pollen grains superior biosensors. Such an architecture is also desirable
in gas-sensing, and thus could be used as the structure
of gas sensors. In our previous work [14,15], we utilized
pollen grains and pollen coats as biotemplates to synthesize MOS gas sensors which faithfully preserved their
porous hierarchical biostructures and exhibited distinguished gas-sensing performances. What’s more, the
doping of carbon in MOS has been reported to promote
gas-sensing properties [16–18], while elemental carbon is
largely contained in pollen grains and can be obtained
directly from pollen biotemplates. Consequently,
MOS/C composites with a porous hierarchical architecture can be fabricated by using pollen grains as biotemplates and have outstanding gas-sensing performances.
In order to retain the architecture of the pollen grain
biotemplates, we use a sol–gel soakage process on them
since this is one of the simplest and most effective ways
to preserve microstructures [19,20]. By the subsequent
calcination of the as-soaked grains, most of the pollen
components can be burnt off and the MOS is formed from
the precursor deposited on the grains. The temperature
1359-6462/$ - see front matter Ó 2013 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.
http://dx.doi.org/10.1016/j.scriptamat.2013.02.018
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F. Yang et al. / Scripta Materialia 68 (2013) 873–876
control of the calcination treatment enables a small
amount of carbon to remain from the biotemplates and
to function as the matrix for the MOS, thereby preserving
the porous hierarchical structure of the pollen grains. As
a result, composites of MOS and C that retain the pollen
grains’ biostructure can be successfully prepared.
a-Fe2O3 is a MOS with a wide energy gap of 2.1 eV,
and has been reported to demonstrate gas-sensing performances [21,22]. Here, pollen grains of Brassica capestris are selected as the templates to fabricate a-Fe2O3/C
composites in situ by the sol–gel soakage method followed by calcination. The porous hierarchical architecture of the composites comes from the pollen grain
scaffold, which is too intricate to be constructed artificially, and demonstrates a large surface area for gassensing reactions and outstanding gas transportability.
Moreover, pollen grains also provide elemental carbon
as a dopant in a-Fe2O3 to enhance the gas sensitivity.
The as-fabricated nanocomposites are thus expected to
show superb gas-sensing performances to noxious gases,
including methanol and acetone, and are promising
materials for future gas sensor applications.
Rape pollen grains (B. capestris) were purchased from
Wangs Co. Ltd. The pollen grains were rinsed in anhydrous ethanol and dried in 60 °C air. Ferric nitrate was
first dissolved into ethanediol to form a 0.1 mol l 1 precursor solution and stirred for 1.5 h. Then 1 g of CTAB
surfactant was added to the precursor solution and the
stir continued for another 0.5 h. After that, the as-rinsed
pollen grains were mixed with the precursor solution by
ultrasonic dispersion for 3 h and soakage for 24 h at
60 °C. The as-immersed pollen grains were then filtered
from the solution, rinsed in ethanol and dried. Finally,
they were calcinated in three consecutive steps, at 100,
250 and 550 °C (2 h). After being cooled down to room
temperature, powders were collected as the final product.
A control sample was synthesized with the same procedure but without the pollen grains as biotemplates.
Thermogravimetric analysis (TGA) was performed
on a TGA2050 analyzer to study the calcination processes
of pollen grains (5 mg) in air. X-ray diffraction (XRD)
measurements were taken on a Rigaku D/max 2550 V
instrument with Cu Ka radiation (k = 1.5406 Å). Fieldemission scanning electron microscopy images (FESEM)
were achieved on a FEI SIRION 200 field-emission gun
scanning electron microscope. Transmission electron
microscopy (TEM) and high-resolution TEM (HRTEM)
patterns were obtained on a JEOL JEM-2100F instrument. Nitrogen adsorption–desorption isotherms were
measured at 77 K on a Micromeritics ASAP 2010M+C
volumetric adsorption analyzer.
Each of the as-fabricated biotemplated a-Fe2O3/C
and Fe2O3 (no templates) powders was mixed with water
and pasted onto an alumina tube attached with two gold
electrodes having a gap of 1 nm. The fittings were then
heated at 500 °C for 1 h and aged at 300 °C for 10 days
for stability. Gas-sensing measurements were carried
out in a Hanwei static test system. In this gas flow apparatus, equipped with an external heating facility, the gas
flow was switched between the target gas and dry air,
while the electrical resistance of the sensor was measured
continuously by an electrometer. The gas response S is
defined as the resistance ratio for reducing gases, Ra/
Rg, where Ra and Rg represent the resistance in dry air
and in the target gas, respectively. The response and
recovery time of the gas sensor are defined as the time taken for S to reach 90% of its total change.
The comparison of TGA results (Fig. S1) between
pollen grains without any treatment and those soaked
in the precursor solution reveals that the as-soaked hybrids lose most of their pollen components in the temperature range from 56 to 500 °C. The hybrids were
thus calcinated to 550 °C to burn off about 95 wt.% of
their organic templates but to preserve an amount of
carbon which is thermally stable at that temperature,
as a dopant [23,24].
Figure S2 demonstrates the XRD pattern of the biotemplated sinter. Most diffraction peaks can be indexed
to Fe2O3 (JCPDS Card No. 33-0664 or 39-0238) and some
can be indexed to graphite C (JCPDS Card No. 65-6212),
indicating the existence of elemental carbon. The carbon
content is calculated to be 14.7 wt.%, which comes directly from the pollen grain biotemplates without the need
to add any extra chemicals. The broad peaks in the XRD
pattern indicate that the biotemplated composite has
small grain sizes. From the Scherrer equation, the average
grain size of a-Fe2O3 is calculated to be 23.6 nm.
The FESEM images in Figure 1 demonstrate the
morphologies of the templated a-Fe2O3/C composite
and original rape pollen grains. The composite particles
are ellipsoidal, with a longitudinal axis of about 12 lm
and a latitudinal axis of around 10 lm (see Fig. 1(a)).
Their surfaces are mesh-like, with various pores, the
sizes of which are uniformly distributed around 0.3–
0.5 lm (see Fig. 1(b)). Although the composite particles
shrank in size from the pollen grains with a longitudinal
axis of about 30 lm and a latitudinal one of around
20 lm due to the burn-off of the pollen nuclei, their
structures are identical (see Fig. 1(a) and (c)). The macropores on the outer surface of pollen grains are about
0.5–1 lm (see Fig. 1(d)). This implies that the precursor
particles containing Fe3+ were first bound to the skeletons of pollen grains, then converted into Fe2O3, and
grew in situ in the process of the pollen components
being burnt off. Carbon remained throughout the
calcination treatment and played the role of a matrix
Figure 1. FESEM images of (a, b) as-synthesized templated a-Fe2O3/C
and (c, d) rape pollen grains: (a, c) full view; (b; d) the outer surface: ,
tectum layer; , columella; , foot layer.
F. Yang et al. / Scripta Materialia 68 (2013) 873–876
Figure 2. (a) TEM image and (b) HRTEM image of templated aFe2O3/C.
Figure 3. Nitrogen adsorption–desorption isotherms. The inset shows
the corresponding pore size distribution of the templated a-Fe2O3/C.
for the Fe2O3 to preserve the pollen grains’ microstructure. The similar three-dimensional porous hierarchical
architectures of the a-Fe2O3/C composite and rape pollen grains are shown in Figure 1(b) and (d). The porous
mesh-like tectum layer is supported by the columellae
that are orderly arranged on the foot layer. The columellae build a large number of funnel-like channels connected by mesopores on the inner scaffolds, forming
interconnective macro-/mesopore networks.
The TEM images in Figure 2 further illustrate the
porous hierarchical structure of the a-Fe2O3/C composite. As is shown in Figure 2(a), the macropores on the
surface are of 0.3–0.5 lm, agreeing with what is observed in FESEM images in Figure 1. The mesopores
on the scaffolds are 30–50 nm in size. A macropore on
the tectum layer and a mesopore on the foot layer can
act as the two ends of a funnel-like channel. Such channels are connected by the mesopores at the interspaces
of columellae into porous hierarchical networks. The
clear lattice fringes in Figure 2(b) indicate the good crystallinity of Fe2O3. The interplanar spacings of 0.231 and
0.272 nm in Figure 2(b) correspond to the {0 0 6} and
{1 0 4} planes of hematite Fe2O3, respectively.
The nanoscale porosity was further studied from the
nitrogen adsorption–desorption isotherm in Figure 3.
The templated a-Fe2O3/C exhibits a type IV isotherm
with a type H3 hysteresis loop, indicating the characteristics of the materials with mesopores. Furthermore, the
large hysteresis loop involving the capillary condensation suggests a large number of interconnected mesopore networks in the a-Fe2O3/C composite, which
strongly confirms the observation of SEM and TEM
images in Figures 1 and 2. Based on the Barrett–
Joyner–Halenda method, the pore volume of the
composite is large, at 0.151 cm3 g 1. The distribution
of the pore size is mostly in a range from 5 to 18 nm,
875
Figure 4. Real-time gas-sensing response curves of templated a-Fe2O3/
C and Fe2O3 (no templates) to (a) methanol and (b) acetone.
with an average of 111.95 Å, while a small number of
macropores exist (see the inset of Fig. 3). Such interconnective pore networks provide huge surface areas for
gas-sensing reactions. According to the Brunauer–Emmett–Teller method, the templated a-Fe2O3/C has a
specific surface area of 55.40 m2 g 1.
The highly interconnective macro-/mesopore networks on the scaffolds retained from the pollen grains
provide superior gas transportability and a large surface
area. With its small grain sizes and multicomponent
characteristics, the biotemplated a-Fe2O3/C has good
potential in gas-sensing performances.
Figure 4 demonstrates the real-time gas-sensing responses of a-Fe2O3/C composite (fabricated by using
pollen grains as biotemplates) and the control sample
(synthesized without using templates) to the reducing
gases methanol and acetone, respectively. The gas
concentrations ranged from 10 to 100 ppm at a working
temperature of 330 °C. As is illustrated in Figure 4, the
gas-sensing response Ra/Rg of the templated a-Fe2O3/C
increased abruptly once the target gases were injected,
stayed stable at a high value and then dropped back
immediately after the gases had been eliminated. The
response and recovery are duplicatable after numerous
cycles. The average response/recovery times of the
a-Fe2O3/C composite to methanol and acetone are 9.5/
7.5 s and 7.5/9.5 s, respectively. Compared with the typical gas response or recovery time of 70–300 s [25–27],
the fast, reproducible response and recovery of the composite prove it to have remarkable gas-sensing ability.
The Ra/Rg responses of the templated a-Fe2O3/C and
the Fe2O3 (no template) increase with elevating gas concentrations. Although the Fe2O3 (no template) also
exhibits response and recovery to the target gases, its
sensitivity is markedly lower than that of templated aFe2O3/C composite. This indicates that the porous hierarchical structure inherited from the pollen grain biotemplates and the doping of carbon cause a clear
enhancement in gas sensitivity. In contrast with various
other MOS-based methanol or acetone sensors with the
morphologies of nanoparticles [28], nanorods [29],
nanotubes [2] and nanofibers [30], the as-prepared porous hierarchical a-Fe2O3/C nanocomposite exhibits
more eminent gas sensitivity.
The prominent methanol and acetone sensing performances can be attributed to three factors. Firstly, since
the a-Fe2O3 average grain size of 23.6 nm is less than
twice the Debye length [31], the gas sensitivity of the
as-synthesized composite is expected to be favorably
high, according to previous literatures [32,33]. Moreover,
the porous hierarchical structure of the a-Fe2O3/C
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F. Yang et al. / Scripta Materialia 68 (2013) 873–876
composite inherited from the rape pollen grains builds
unique macro-/mesopore networks for gas-sensing. The
macropores on the tectum layer of a composite particle
function as entrances to its interior for mass gas transportation, while the columellae construct funnel-like
channels for the swift diffusion of gas molecules (see
Figs. 1(b) and 2(a)). The inner mesopores on the composite scaffolds connect the macropores on the surface,
making the whole particle tremendously interconnective.
The templated a-Fe2O3/C thus has a large pore volume
of 0.151 cm3 g 1 and a huge specific surface area of
55.40 m2 g 1. Consequently, gases are able to move at
a high rate and volume in the nanostructure, and have
gas-sensing reactions at a large number of sites on its surface, leading to superb gas-sensing performances. Lastly,
as the main component of the pollen remnants after calcination, carbon plays the role of a matrix for Fe2O3 and
preserves the porous hierarchical structure of pollen
grains. Gas sensitivity has been reported to be strongly
enhanced when carbon is doped in MOS [16–18], and
one plausible explanation is that carbon can function
as shallow donors to semiconductors [34]. Besides, the
mechanical and thermal stability of carbon enable the
structure of the composite to be highly stable [23]. All
these factors make the as-prepared a-Fe2O3/C nanocomposite a superior gas sensor with a stable architecture.
In summary, a biokindled strategy has been demonstrated to fabricate a-Fe2O3/C composites with a porous
hierarchical nanostructure by using rape pollen grains as
biotemplates. The pollen grains were soaked in precursor solution and calcinated to preserve their intricate
architecture, which enables good gas transportability
and an enormous specific surface area (55.40 m2 g 1)
for gas-sensing reactions. Fe2O3 has small-scaled nanocrystallites, while carbon helps to render the composite
a good, stable gas sensor. As a result, not only did the
a-Fe2O3/C composite exhibit fast responses and recoveries to noxious methanol and acetone, but it also had
high sensitivity and favorable reproducibility to the target gases. These results prove that the biotemplated aFe2O3/C composite is remarkable among gas-sensing
materials, and is promising for use in gas sensor applications for environmental gas detection. The illustrated
method explores a new means of preparing nanocomposites that have porous hierarchical structures with a
high intricacy. Various biotemplates with such structural hierarchy can be potentially utilized to fabricate
functional materials for applications in catalysts, electronic devices, filters, et al. in the future.
Financial support from the National Natural
Science Foundation of China (Grant No. 51102165,
51131004), the 973 National Project (Grant No.
2011CB922200), Shanghai Science and Technology
Committee (Grant No. 10JC1407600) and the Research
Fund for the Doctoral Program of Higher Education of
China (Grant No. 20120073130001) is gratefully
acknowledged. We also thank Prof. Jiaqiang Xu from
Shanghai University for the gas-sensing test.
Supplementary data associated with this article can
be found, in the online version, at http://dx.doi.org/
10.1016/j.scriptamat.2013.02.018.
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