Synthesis and gas-sensing characteristics of x-Fe2O3 hollow balls

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Original article

Synthesis and gas-sensing characteristics of

a-Fe

₂

O

₃

hollow balls

Chu Manh Hung, Nguyen Duc Hoa

**

, Nguyen Van Duy, Nguyen Van Toan,

Dang Thi Thanh Le, Nguyen Van Hieu

*

International Training Institute for Materials Science, Hanoi University of Science and Technology, No. 1 Dai Co Viet Street, Hanoi, Viet Nam

a r t i c l e i n f o

Article history:
Received 7 March 2016
Accepted 21 March 2016
Available online 11 April 2016
Keywords:

a-Fe2O3hollow balls

Hydrothermal
Gas sensors

a b s t r a c t

The synthesis of porous metal-oxide semiconductors for gas-sensing application is attracting increased
interest. In this study, a-Fe2O3 hollow balls were synthesized using an inexpensive, scalable, and

template-free hydrothermal method. The gas-sensing characteristics of the semiconductors were
sys-tematically investigated. Material characterization by XRD, SEM, HRTEM, and EDS reveals that
single-phasea-Fe2O3hollow balls with an average diameter of 1.5mm were obtained. The hollow balls were

formed by self assembly ofa-Fe2O3nanoparticles with an average diameter of 100 nm. The hollow

structure and nanopores between the nanoparticles resulted in the significantly high response of thea
-Fe2O3hollow balls to ethanol at working temperatures ranging from 250
C to 450
C. The sensor also

showed good selectivity over other gases, such as CO and NH3promising significant application.

© 2016 Publishing services by Elsevier B.V. on behalf of Vietnam National University, Hanoi. This is an
open access article under the CC BY license ( />

1. Introduction

Chemical and gas sensors are attracting increased interest
worldwide because of the growing demand for monitoring gaseous
molecules in various applications [1,2]. Various wide-bandgap
metal oxide semiconductors, such as SnO2, TiO2, ZnO, In2O3,

Fe2O3, and WO3, have been synthesized for gas-sensing

applica-tions[3]. The synthesis of earth-abundant metal oxides, such as
Fe2O3, with a three-dimensional configuration and a porous

structure for advanced applications has been the topic of interest in
recent years [4e6].

a

-Fe2O3 is a nontoxic, stable, and

earth-abundant transition metal oxide [7,8]. This compound has been
used as a sensing material for the detection of various gases[9],
such as CO[10], xylene[11], and acetone[12], among others[13]. A
hollow spherical structure has been reported to show significantly
faster response and recovery times, as well as higher response to
analytic gases, compared with other structures. Thus, recent studies

have focused on the synthesis of this material for sensing

applications [14,15]. Hollow balls are typically fabricated by a
template-assisted method, in which the scarified template is
pre-pared_{first, then the desired materials are coated, and finally the}
template is removed [16]. For instance, hollow sphere Fe2O3

composed of ultrathin nanosheets were prepared by

template-assisted method, in which monodispersed Cu2O spheres were

used as scarified template to synthesize FeOOH, which was
sub-sequently converted into

a

-Fe2O3nanospheres[17]. Wang et al.[15]

prepared Fe2O3 hollow spheres using ZnS-cyclohexylamine as a

template-assisted agent. However, the use of template in the
syn-thesis of hollow balls has some limitations, such as multiple-step
processes and contamination by foreign elements[17].

In this study, we synthesized

a

-Fe2O3 hollow balls using a facile,

inexpensive, and scalable hydrothermal method using glucose and ferric
chloride hexahydrate as precursors for gas-sensing applications. The

a

-Fe2O3hollow balls were formed by the aggregation of single-crystal

a

-Fe2O3nanoparticles with an average diameter of 100 nm. The

inter-space between aggregated nanoparticles facilitates the entry of the gas
molecules into the hollow balls and adsorption on the total surface of

the

a

-Fe2O3nanoparticles, thus enhancing the sensing performance.

2. Experimental

Large-scale

a

-Fe2O3hollow balls were synthesized using a facile

and template-free hydrothermal method with glucose and ferric
chloride as precursors. In a typical synthesis, 2.7 g of ferric chloride
hexahydrate (99%, SigmaeAldrich) and 3.7 g of glucose (99.5%,
* Corresponding author. International Training Institute for Materials Science

(ITIMS), Hanoi University of Science and Technology (HUST), No.1, Dai Co Viet Road,
Hanoi, Viet Nam. Tel.:ỵ84 4 38680787; fax: ỵ84 4 38692963.

** Corresponding author. International Training Institute for Materials Science
(ITIMS), Hanoi University of Science and Technology (HUST), No.1, Dai Co Viet Road,
Hanoi, Viet Nam. Tel.:ỵ84 4 38680787; fax: ỵ84 4 38692963.

E-mail addresses:(N.D. Hoa),(N. Van
Hieu).

Peer review under responsibility of Vietnam National University, Hanoi.

Contents lists available atScienceDirect

Journal of Science: Advanced Materials and Devices

j o u r n a l h o m e p a g e : w w w . e l s e v i e r . c o m / l o c a t e / j s a m d

/>

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SigmaeAldrich) were dissolved in 50 ml of deionized water at room
temperature to obtain a clear solution. Then, ammonium hydroxide
(25%) was added dropwise to adjust the pH level to pH 7 to obtain a
milky solution. The milky solution was then poured into a Te
flon-lined autoclave for hydrothermal treatment at 180
C for 24 h
before cooling to room temperature naturally. The precipitates
were washed several times with deionized water and ethanol and
then collected by centrifugation at 4000 rpm. Finally, the collected
products were air dried at 60
C for 24 h and then calcined at 600
C
for 4 h prior to use in sensor fabrication and characterizations. The
morphology and crystal structure of the synthesized materials
were characterized usingfield-emission scanning electron
micro-scopy (FESEM, JSM, 7600F), transmission electron micromicro-scopy
(TEM, Tecnai, G20, 200 kV, FEI), and X-ray powder diffraction (XRD,
Bruker D8 Advance)[18].

The gas sensor was fabricated by dispersing the obtained
powders in dimethyl formamide solution and then coating the
mixture onto a pair of comb-type Pt electrode deposited on
ther-mally oxidized silicon substrate. The gas-sensing characteristics

were measured by aflow-through technique with a standard flow

rate of 400 sccm for both dry air and balanced gas using a
home-made sensing system. Details of the sensing system can be found in
our recent publication[19]. Gas-sensing characteristics were tested
using ethanol, CO, and NH3at temperatures ranging from 250
C to

450
C. The sensor response S was defined as S ¼ Rair/Rgas for

reducing gases, where Rgasand Rairare the sensor resistances in the

presence of test gas and dry air, respectively.

3. Results and discussion
3.1. Material characterization

The morphology of the synthesized materials was characterized
by FESEM [Fig. 1]. The as-hydrothermal products have a spherical
shape with an average diameter of approximately 1.5

m

m [Fig. 1(A)
and (B)]. The glucose may have decomposed and grown into carbon
spheres under the hydrothermal treatment[20]. The ferric particles
were then aggregated on the surface of carbon spheres to form the
coreeshell materials[21]. The carbon cores were burned out after
calcination at 600
C, forming the

a

-Fe2O3 hollow spheres

[Fig. 1(C)e(F)]. The

a

-Fe2O3 hollow balls were formed from the

aggregated nanoparticles with an average diameter of 100 nm. The
shell of the hollow balls is not a dense material, but porous as a
result of the nanoparticle aggregation. The shell thickness of the
hollow sphere from the broken area is estimated to be
approxi-mately one layer of nanoparticles [Fig. 1(E)].

TEM images and elemental analytical results by EDS of the

a

-Fe2O3hollow balls are shown inFig. 2. The hollow structure of the

a

-Fe2O3balls is clearly shown inFig. 2(A), in which the central part

is brighter than the surrounding region. The HRTEM image of the
sample demonstrates the high crystallinity of the

a

-Fe2O3phase

where the gap between two adjunction fringes is approximately
0.25 nm, corresponding to the interspace of (110) planes[22]. The
inter-grain boundary between nanoparticles can also be seen in the
HRTEM image. Selective area electron diffraction of the selected

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area marked by white square in the HRTEM image exhibits
diffraction spots revealing the single crystallinity of

a

-Fe2O3. The

EDS analytical results of the sample shown inFig. 2(C) demonstrate
the peaks of C, O, Fe, and Cu. Elements C and Cu came from the
carbon-coated Cu grid used for TEM characterization, whereas O
and Fe were from the sample. The ratio [O]/[Fe]¼ 1.64 is higher
than the composition of stoichiometric Fe2O3, possibly because of

contamination of some OH groups on the surface of sample.
Crystal structures of the as-hydrothermal and calcined materials
characterized by XRD are shown inFig. 3(A) and (B), respectively.
The XRD patterns of the as-hydrothermal product shown in

Fig. 3(A) illustrate unresolved peaks. The metastable phase, such as
Fe(OH)2or Fe(OOH), may have been formed after the hydrothermal

process[6]. The metastable phase was then converted to Fe2O3by

thermal oxidation at high temperature. The XRD pattern of the
calcined sample [Fig. 3(B)] demonstrates that the materials have a
rhombohedral crystal structure, with the main peaks indexed to the
standard profile of

a

-Fe2O3phase (JCPDS No. 86e0550)[22]. No

detectable peaks of FeOOH or Fe3O4impurities and other phases

were observed, indicating the formation of single-phase

a

-Fe2O3.

No template was used in the fabrication of hollow balls, thus the
products were not contaminated by any foreign element[15].
3.2. Gas-sensing characteristics

Gas-sensing characteristics of the synthesized

a

-Fe2O3hollow

balls were tested using ethanol at different temperatures ranging
from 250
C to 450
C [Fig. 4]. Fig. 4(A) shows that the initial
resistance of the

a

-Fe2O3 hollow ball sensor measured in air at

250
C, 300
C, 350
C, 400
C, and 450
C were approximately 85,
58, 43, 31, and 18 k

U

, respectively. The decrease in the initial
resistance of

a

-Fe2O3sensor with increasing operating temperature

reveals the semiconducting nature of metal oxide, that is, the
thermal energy excites electrons from valence band to conduction

band to contribute to the conductivity of the material[23]. The

a

-Fe2O3hollow balls showed n-type semiconducting characteristics

at all measured temperatures. The sensor resistance decreased
significantly upon exposure to reducing gases (ethanol, NH3, and
Fig. 2. (A, B) Transmission electron micrographs and (C) EDS results ofa-Fe2O3hallow balls; inset of (B) is the corresponding FFT.

Fig. 3. X-ray diffraction patterns of the (A) as-hydrothermal and (B) calcineda-Fe2O3

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CO) [6]. The semiconducting characteristics of metal oxide are
determined by the deficiency or excess of material composition.
The excess or deficiency of oxygen in the crystal structure of

a

-Fe2O3generally leads to p-type or n-type semiconducting

charac-teristics[24,25]. Long et al.[26]demonstrated that the polyhedral

a

-Fe2O3 particles showed p-type gas-sensing characteristics, in

which the sensor resistance increased upon exposure to reducing
gases, such as H2, CO, and C2H5OH. They reported that the p-type

characteristic of materials was due to the incorporation of Na into

a

-Fe2O3oxide. Heat treatment temperature significantly influences

the electrical properties of

a

-Fe2O3, such that high-temperature

treatment can result in p-type characteristics[25]. In this study,
the synthesized

a

-Fe2O3hollow balls were heat-treated at a

rela-tively low temperature of approximately 600
C for 4 h, so the balls
exhibited n-type characteristics. This result is consistent with other
reports, where the n-type nature of metal oxide semiconductor was
attributed to the presence of oxygen vacancies[12,27]. The effect of
temperature heat treatment on the ethanol-sensing characteristics
of

a

-Fe2O3hollow balls was determined by annealing the sample at

800
C for 2 h. However, high-temperature heat treatment led to
the distortion of sensor response [Fig. S1, Supplementary]. Sensor

response as a function of ethanol concentration measured at
different temperatures is shown inFig. 4B. The sensor response
increases with increasing working temperature from 250
C and

reaches a maximum value at 400
C. Further increase in the

working temperature results in a slight decrease in the sensor
response. At 400
C, the sensor response also increases from 1.77 to
4.29 with increasing ethanol concentration from 50 ppm to
500 ppm. Fast response and recovery times of the sensor are also
important in real-time measurements of the device[18]. The 90%

response and recovery times of the sensor at different

Fig. 4. Ethanol-sensing characteristics ofa-Fe2O3hollow balls: (A) transient resistance versus time of sensor upon exposure to different concentrations of ethanol at various

temperatures; (B) sensor response as functions of ethanol concentrations, (C) response and recovery times; (D) short-term stability of sensors.

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temperatures were calculated [Fig. 4(C)]. The response and
recov-ery times to 50 ppm ethanol were approximately 16/30, 7/20, 4/15,
and 3/12 s at working temperatures of 250
C, 300
C, 350
C,
400
C, and 450
C, respectively. The response and recovery times
decrease with increasing working temperature because of the
ac-celeration of thermal energy for the adsorption and desorption
processes[18]. The fast response recovery times of less than 1 min
is sufficient for practical application[27]. The transient stability of
the fabricated sensor was also tested at 450
C for several cycles
switching from air to analytic gas and back to air [Fig. 4(D)]. A slight
deviation from the baseline resistance is observed after several
cycles possibly due to the poor adhesion of sensing layer and

substrate. The experiment was repeated for a day, and negligible
distortion in response was found, indicating suf_{ficient stability.}

Selectivity of the sensor to CO and NH3 was also tested at

different temperatures [Fig. 5(A) and (B), respectively]. The sensor

resistance decreased upon exposure to CO and NH3 gases. The

response and recovery characteristics to CO gas improve with
in-crease in temperature. At 450
C, the sensor response to 25, 50, and
100 ppm CO is very low, that is, approximately 1.22, 1.30, and 1.33,

respectively. The sensor also shows weak response to NH3

(50÷500 ppm) [Fig. 5(B)]. At 450
C, the sensor responses to 50, 100,
250, and 500 ppm NH3are 1.04, 1.11, 1.18, and 1.27, respectively. The

response of

a

-Fe2O3hollow balls to ethanol (500 ppm) is 3.38 times

higher than that to NH3(500 ppm), at low working temperature,

suggesting the possibility of using this material for sensing ethanol.
3.3. Gas-sensing mechanism

The gas-sensing mechanism of the fabricated sensor can be
explained by the spaceecharge layer mode[28]. The gas-sensing
characteristics were measured under a continuousflow of dry air.
Thus, the oxygen molecules in air can capture the free electron from

a

-Fe2O3crystals to form the electron-depletion region. The oxygen

molecules adsorb on the surface of the sensing layer in the form of
O_{2 ,}Oand O2_{, as follows}_[29]_.

O2gasị ỵ e4O2adsị (1)

O₂gasị ỵ e_42O_adsị ₍₂₎

Oadsị ỵ e4O2 (3)

The analytic molecules interact with the pre-adsorbed oxygen
upon exposure to ethanol gas, according to the following
equations:

C2H5OHỵ 3O242CO2ỵ 3H2Oỵ 3e (4)

C2H5OHỵ 6O42CO2ỵ 3H2Oỵ 6e (5)

C2H5OHỵ 6O242CO2ỵ 3H2Oỵ 12e (6)
The interactions between analytic ethanol molecules and
pre-adsorbed oxygen release electrons back to the crystals and reduce
the space_{echarge layer, resulting in decreased sensor resistance.}
The porosity of the sensing layer is also very important in
con-trolling the sensitivity of the device because it decides the diffusion
rate of analytic gas molecules into the sensing layer. The diffusion
constant (DK) can be calculated based on the Knudsen diffusion

model as DK¼ 4r/3(2RT/

p

M)1/2, where r is the pore size, R is the

universal gas constant, T is the temperature, and M is the molecular
weight of the diffusing gas[30]. In this study, the shell of the hollow
balls was formed by the aggregation of the monolayer
nano-particles with approximately 100 nm in diameter. The interspace

between nanoparticles acted as diffusion path for analytic gas
molecules to enter deeply into the balls to be adsorbed on the total
surface of sensing materials, thereby enhancing sensing
perfor-mance[31].

4. Conclusion

The synthesis of

a

-Fe2O3hollow balls by a facile hydrothermal

method for gas-sensing application is introduced. The

a

-Fe2O3

hollow balls were formed by the aggregation of highly crystalline

a

-Fe2O3 nanoparticles. The average diameters of

a

-Fe2O3

nano-particles and hollow balls were 100 nm and 1.5

m

m, respectively.
The interspace between nanoparticles and hollow structure of the
materials facilitate the fast diffusion of analytic gas molecules into
the sensing layer and adsorption on the total surface of sensing
materials. These characteristics ensured the high sensitivity of
materials. Thus, the

a

-Fe2O3hollow balls were found to be suf

fi-cient for ethanol sensor application.
Acknowledgment

The present study was funded by the Vietnam Ministry of

Ed-ucation and Training under Code No. KB2015e01e100.

Appendix A. Supplementary data

Supplementary data related to this article can be found athttp://
dx.doi.org/10.1016/j.jsamd.2016.03.003.

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