World Journal of Engineering
VOLATILE ORGANIC COMPOUND GAS SENSOR BASED ON MESO-/MACRO
POROUS SEMICONDUCTING METAL OXIDE
Nguyen Duc Hoa,1 Sherif A. El-Safty1*
Materials Research Laboratory for Environmental and Energy, National Institute for Materials Science
(NIMS), 1-2-1 Sengen, Tsukuba-shi, Ibaraki, 305-0047, JAPAN. E-mail: sherif.elsafty@nims.go.jp
Introduction
synthesis, different amount of urea and 0.01 mol of
cobalt chloride were dissolved in a 50 ml distilled
water to form a transparent solution. The ratio
r=[Urea]/[CoCl2] was 0.5, 1, 2, and 4. The
hydrothermal process was carried at 180 oC for 16 h
and then allowed to cool at room temperature. After the
hydrothermal process, the precipitate was collected,
and subsequently calcinated at 500 oC for 8 h to obtain
Co3O4. The gas sensors were fabricated using a thick
film technique, where the sensing powers were mixed
with organic binders, and then pasted onto a substrate
supported digital Pt/Ti electrodes. The electrical
resistance of sensors was measured in a dynamic
condition, in which the dry air and the analytical gases
continuously flowed through the sensing chamber, at a
total flow rate of 500 cm3/min during the
measurements. The sensor response S is defined
S(%)=100×(R-R0)/R0, where R and R0 are sensor
resistances in the presence of analytical gases and dry
air, respectively.
In the past few years, the investigations about the
gas sensors have been received great attentions
because of their diverse applications, include
environmental monitoring, transportation, security,
defense, space missions, energy, agriculture, and
medicine [1]. Recent research is focused on the
development of metal oxide semiconducting
materials with new structures or morphologies to
improve sensitivity, selectivity, and stability of gas
sensors [2]. The mesoporous materials are of
interest due to their high specific surface area and
unique properties [3]. The use of mesoporous
structures leads to improve the gas sensors
performance throughout the enhancement of total
volume exposure of the gas targets. Despite that,
the synthesis of mesoporous semiconducting metal
oxides by the conventional organic template
pathways is difficult because the mesostructure
pores tend to collapse during the removal of the
organic template at high temperature treatments
[4].
Results and Discussion
The crystal structures of materials investigated by
XRD patterns are shown in Fig. 1. The
hydrothermal product exhibits typical diffraction
peaks of orthorhombic cobalt basic carbonate
Co(OH)x(CO3)0.5.0.11H2O (JCPDS, No. 48-0083)
phase [Fig. 1(A)]. The XRD pattern of calcinated
product was indexed at the typical diffraction
peaks of cubic Co3O4 (JCPDS, No. 42-1467) [Fig.
1(B)]. There was no peak of other impurities
observed, indicating the formation of single-phase
Co3O4.
In this paper, we report on the synthesis of meso/macro porous Co3O4 for gas sensor applications.
Different geometries of Co3O4 are fabricated by a
facile hydrothermal method without using any
surfactant or structure-directing agent. The gas
sensing properties of synthesized materials are
investigated for VOCs detection.
Experimental
The meso-/macro porous Co3O4 materials were
synthesized by a hydrothermal method [5]. In a typical
861
World Journal of Engineering
acetone, followed by ethanol and benzene. The
sensor response was 1850%, 2236%, and 2664%
for 74570, 149111, and 223666 ppm concentration
of acetone, respectively. The linear dependence of
sensor response as a function of acetone
concentration in measured range is one of the
advances in its practical application because of the
easy design in the read-out signal circuit
Fig. 1 XRD patterns of (A)- hydrothermal
products, and (B)- after calcination.
The effects of urea content on the morphologies of
Co3O4 investigated by FE-SEM images are shown
in Fig. 2 (A)-(D). At low ratio of r=0.5, the
product contains nanoparticles with grain size of
~100 nm [Fig. 2(A)]. Increasing the r value to 1 or
2, the obtained products were porous nanorods in
which the nanorods are generated from aggregated
nanoparticles of average particles size is about 100
nm, as shown in Fig. 1 (B). The porous nanorods
have an average diameter of about 250 nm, and a
length up to micrometers. However, further
increase the r value to 4, the porous plates were
obtained instead of porous nanorods [Fig. 1(D)].
Fig. 3 (A) the dependence of sensor response on
the [Urea]/[CoCl2] ratio, and (B) the response of
porous Co3O4 nanorods as a function of gases
concentration.
Conclusion
Different morphologies of porous Co3O4 materials
have been successfully synthesized. The
synthesized materials were found to be effective in
the detection of acetone, benzene, and ethanol. It
has good stability, high sensitivity, and fast
response and recovery time.
Acknowledgment
This work was supported by the Japan Society for
the Promotion of Science (JSPS), grant No.
P09606.
Fig. 2 FESEM images of Co3O4 synthesized with
different r=[Urea]/[CoCl2] ratio: (A) r=0.5, (B)
r=1, (C) r=2, and (D) r=4.
References:
1. Sysoev, V. V., Goschnick, J., Schneider T., Strelcov,
E., Kolmakov, A., Nano Lett., 7 (2007) 3182-3188.
2. Wang, Y. D., Djerdj, I., Antonietti, M., Smarsly, B.,
Small, 4 (2008) 4, 1656-1660.
3. (a) S. A. El-Safty, et al. Phys. Chem. C, 112 (2008)
4825-4835; (b) S. A. El-Safty, T. Hanaoka, Adv.
Mater. 15 (2003) 1893-1899; (c) S.A. El-Safty, T.
Hanaoka and F. Mizukami, Adv. Mater. 17 (2005)
47.
4. Li, L., Krissanasaeranee, M., Pattinson, S. W.,
Stefik, M., Wiesner, U., Steiner, U., Eder, D., Chem.
Commun., 46 (2010) 7620-7622.
5. Nguyen, H., El-Safty, S. A., J. Phys. Chem. C, 115
(2011) 8466-8474.
The gas sensing properties of materials
synthesized with different [Urea]/[CoCl2] ratio
were investigated for benzene, as shown in Fig.
3(A). It is clearly that the material synthesized at
[Urea]/[CoCl2] ratio of r=2 showed the highest
response. This mean that the nanorods based
sensor has the highest response. The sensing
properties of porous nanorods were further
investigated for detection of acetone, benzene, and
ethanol [Fig. 3(B)]. Sensor response is highest for
862
World Journal of Engineering
861