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Gas sensing property and microstructure of SnO2 nanocrystallineprepared by solid state reaction*/thermal oxidation
Yue Chen, Jianmin Zhu *, Xinhua Zhu, Guobin Ma, Zhiguo Liu, Naiben Min
National Laboratory of Solid State Microstructures, Department of Physics, Nanjing University, Nanjing 210093, People’s Republic of China
Received 14 June 2002; received in revised form 17 September 2002
Abstract
Nanocrystalline SnO2 powders have been synthesized by two-step solid state reaction technique. Firstly, the brown SnO particles
were obtained by grinding mixed SnCl2 �/2H2O and KOH powders at room temperature. Then, the powders were calcined in air
(thermal oxidation) to form the SnO2 nanoparticles. The phase compositions and microstructures of the product were examined by
thermogravimetry-differential thermal analysis (TG-DTA), X-ray diffraction (XRD) and high resolution transmission electron
microscopy (HRTEM), respectively. Furthermore, we have measured the gas-sensing property of the products and found its distinct
selectivity towards ethanol at the presence of gasoline and acetylene, which is different from the property of pure SnO2
nanoparticles. The mechanism of such a specificity was discussed briefly.
# 2002 Elsevier Science B.V. All rights reserved.
Keywords: Gas sensing property; SnO2; Microstructure
PACS numbers: 71.24.�/q; 61.14.Lj; 81.20.Ev
1. Introduction
Since the semiconductor gas sensors were first re-
ported in 1962 [1], these devices have been subjected to
extensive research for detecting small amounts of
inflammable, toxic gases in air. Among them SnO2-
based gas sensors were mostly noticed because of their
high sensitivity, low cost, fast response speed, and low
power consumption. However, there are also some
problems in SnO2-based sensors, of which a serious
one is the relative lack of selectivity [2], since the
chemisorbed oxygen (responsible for controlling surface
conductivity) reacts with a wide range of reducing gases.
Up to date, the most popular methods to obtain stable
specificity are the addition of catalysts or doping
materials (usual transitional metals) into the pure
SnO2 sensing materials [3�/5]. For example, Bulpitt
and coworkers obtained selectivity towards C4 hydro-
carbon gases by addition of Pd [6], and Coles et al. gotselectivity to H2 and CO by the addition of different
amount of Bi2O3 [2]. However, the doping method is a
relatively complex preparing process.
In this work, we present a new method for synthesiz-
ing ethanol-sensitive SnO2 sensing material, which has
few been reported to our knowledge. The SnO2 nano-
particles were prepared by two-steps: solid state reac-
tion, followed by thermal oxidation. We have measuredthe gas-sensing property of the SnO2 nanocrystalline,
and the results show that the products have distinct
selectivity towards ethanol at the presence of gasoline
and acetylene. That indicates the new method being
more convenient to obtain the specificity compared with
the conventional doping method. The microstructures
and phase compositions of the products were also
investigated by thermogravimetry-differential thermalanalysis (TG-DTA), X-ray diffraction (XRD) and high
resolution transmission electron microscopy (HRTEM),
respectively. The possible origin of the selectivity to
ethanol of the products is discussed briefly.
* Corresponding author. Tel.: �/86-25-359-2772; fax: �/86-25-359-
5535.
E-mail address: jmzhu@nju.edu.cn (J. Zhu).
Materials Science and Engineering B99 (2003) 52�/55
www.elsevier.com/locate/mseb
0921-5107/02/$ - see front matter # 2002 Elsevier Science B.V. All rights reserved.
doi:10.1016/S0921-5107(02)00569-X
2. Experiments
SnO2 nanoparticles were synthesized by two-step solid
state reaction method. First, mixed solid powders ofSnCl2 �/2H2O (0.01 mol) and KOH (0.02 mol) were
grinded and ground for 30 min at room temperature.
The reaction started immediately during the mixing
process. The products were washed with distilled water,
treated in an ultrasonic bath and centrifuged. After
being dried, the brown SnO powders (assigned S1) were
obtained. Then the S1 was calcined in air for 2 h at 100,
150, 200, 300, 400 and 600 8C, respectively. The color ofthe products turned from brown to pale, indicating its
oxidization into SnO2. The reactions can be expressed
as:
SnCl2 � 2H2O(s)�2KOH(s)
�2KCl(s)�SnO(s)�3H2O(g) (1)
2SnO(s)�O2(g)�2SnO2(s) (2)
The TG-DTA of the product was performed in N2
and O2 by a TA-Inst 2100 thermal analyzer at a heating
rate of 20 8C min�1 from room temperature to
1000 8C. To investigate whether there are Sn2� cations
in the system, the products were treated with KMnO4
solution, then analyzed with the spectrophotometric
method at 525 nm. XRD patterns of the products in
different conditions were obtained by a XD-3A dif-
fractometer with Cu Ka radiation (l�/1.5418 A). Andthe microstructures of the products were examined by
high resolution transmission electron microscope (JEM-
4000EX). The sensing property of the SnO2 products
was measured through the gas sensors made in conven-
tional way [7]. In order to improve their stability and
repeatability, the gas-sensing elements were sintered at
600 and 800 8C, respectively, for 24 h in air prior to use.
3. Results and discussion
Fig. 1 shows the TG-DTA curves of S1. The total
4.4% weight loss of S1 in N2 (Fig. 1a) was much less
than the theoretical weight loss (11.8%) of Sn(OH)2 to
SnO, suggesting that the SnO is the main composition of
S1 due to the decomposition of Sn(OH)2 during thepreparing process. It is also observed in Fig. 1b that the
SnO can be oxidized to SnO2 easily in air at about
300 8C. But based on the analysis of the spectrophoto-
metric method, it is believed that some unoxidized Sn2�
cations still exist in this system even at relatively high
temperatures (about 0.21 and 0.12% at 400 and 600 8C,
respectively, as we deduced).
Fig. 2 shows the XRD patterns of the products indifferent conditions. The three marked peaks (as shown
in Fig. 2a and b) attributed to SnO phase (JCPDS no. 6-
395) indicate that SnO particles with high degree of
crystallinity have formed in Eq. (1), and they are still
stable up to 200 8C. After calcination at 400 and
600 8C, the (101) diffraction peak of SnO disappeared,
Fig. 1. The TG-DTA curves of S1 in (a) N2, the endothermic peak at
about 80 8C together with a shoulder peak at 160 8C is due to the loss
of surface and structure water. The exothermic peak at about 350 8Cmay be caused by the very little O2 in N2, which results in the oxidation
of SnO. And the exothermic peak at about 580 8C is caused by the
decomposition of SnO particles. The total weight loss of the sample is
4.4%. (b) Air, the big endothermic peak at about 300 8C, accompany-
ing with a weight increase, shows the rapid oxidation of SnO to SnO2.
The weight of the system increases about 9% and becomes constant at
600 8C.
Fig. 2. XRD patterns of S1 (a) as-prepared, (b) calcined for 2 h at
200 8C, (c) 400 8C, (d) 600 8C.
Y. Chen et al. / Materials Science and Engineering B99 (2003) 52�/55 53
and new peaks (Fig. 2c and d) come from SnO2 of
cassiterite (JCPDS no. 41-1445) were observed. This was
attributed to the thermal oxidation of SnO into SnO2.
The transmission electronic microscopy (TEM) image
of the as-prepared product (S1) is shown in Fig. 4a, in
which most grains have diameters of several hundred
nm, yet particles with size of more than 1 mm also exist.
The unit cells of SnO (Sys. Tetragonal, S. G. P4/nmm)
and SnO2 (Sys. Tetragonal, S. G. P42/mnm) lattice are
displayed in Fig. 3. The selected area electronic diffrac-
tion (SAED) pattern (Fig. 4b) presents spots of the [001]
zone-axis of SnO lattice, which is in agreement with the
XRD results. When the calcined temperature of S1
increased, the SnO2 phase was formed, then, the amount
of this phase increased with increasing of the tempera-
ture. This was confirmed by the SAED pattern, as
shown in Fig. 5b and Fig. 6b: when calcined at 150 8C,
the diffracted rings of SnO2 phase were very weak,
whereas it became clear polycrystalline rings after
calcined at 300 8C, disclosing the nearly complete
oxidation of SnO. The HRTEM micrograph at 300 8C(Fig. 6a) shows the SnO2 polyhedron nanoparticles with
mean size of about 5 nm. Then, after calcinations at 400
and 600 8C, the average particle size increased from
about 7.8 to 12.5 nm.
The thermal oxidation process of SnO may be
described as following: first, as the existing of environ-
mental oxygen, SnO2 phase nucleated and formed
dispersed clusters on the surface of SnO particles. As
the increase of calcining temperature, the SnO2 clusters
kept on growing into nanoparticles and the SnO
powders were consumed gradually, until almost de-
pleted. We will discuss this process in details in future
work.
The results of the measurement of gas-sensing prop-
erty listed in Table 1 indicate that the sensors have high
selectivity and sensitivity to ethanol gas in the presence
of gasoline and acetylene (the maximum value of
selective coefficient achieves 11.8). To our knowledge,
this result has rarely been reported in the literature. As a
comparison, we measured the sensing property of pure
SnO2 nanoparticles prepared by one-step solid state
reaction and did not find such selectivity. So the
selectivity may be ascribed to the remained Sn2�
cations, whose existence were confirmed by the spectro-
photometric method, but not revealed by XRD and
SAED because of the small concentration.
The possible mechanism can be explained as follow-
ing: To keep the charge balance in the SnO2 products,
some more oxygen vacancies (V ƒo) should exist because
of the existence of Sn2� cations replacing the Sn4� ions.
So when the SnO2 nanoparticles were exposed in ethanol
gas, the oxygen vacancies reacted with the OH� in
ethanol molecule and OH free radicals were produced at
the surface layer [8], as shown in Eq. (3).
V ??o�2OH��2OH (3)
These free radicals changed the height of the surface
barrier of the particles and, correspondingly, the mate-
rial’s electrical conductance. As for gasoline and acety-
lene, the V ƒo do not oxidize them because of the lack of
OH�.Fig. 3. The unit cells of (a) SnO and (b) SnO2.
Fig. 4. (a) TEM graph and (b) SAED pattern of as-prepared S1, the
(000), (1�//
��1
�/0) and (110) reflections are pointed out.
Fig. 5. (a) TEM graph of S1 calcined at 150 8C. (b) Corresponding
SAED pattern. (c) SAED after computer processing to give a clearer
image, the diffraction spots and rings indicate SnO and SnO2 phases,
respectively.
Y. Chen et al. / Materials Science and Engineering B99 (2003) 52�/5554
4. Summary
SnO2 nanoparticles were synthesized by solid state
reaction-thermal oxidation, and their gas-sensing prop-
erty and microstructures were also investigated. The
material exhibits different gas-sensing property from
pure SnO2 and is sensitive to ethanol at the presence of
gasoline and acetylene. Such selectivity can be useful inpractical application as a cheap, convenient and non-
pollution detecting method.
Acknowledgements
We acknowledge Feng Li and Jiaqiang Xu for the
offering of the samples and the help in the measurementof gas-sensing property. This work was supported by the
National Natural Science Foundation of China and
Opening Project of National Laboratory of Solid
Microstructures, and a Grant for State the Key Program
for Basic Research of China.
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Fig. 6. (a) HRTEM graph of S1 calcined at 300 8C (b) corresponding SAED pattern, the weak ring marked with an arrow indicates the remained
SnO phase ((101) face, the lattice distance is calculated to be 2.90 A), and the three rings marked with numbers attribute to SnO2 (110), (101) and
(211) reflections.
Table 1
The gas-sensing property of the final products, the concentration of all tested gases is 100 ppm
Sintering temperature (8C) Operating temperature (8C) Electric resistance (MV) Selective coefficient
Rair Rethanol Rgasoline RC4H10 Kethanol/gasoline Kethanol/C4H10
600 350 4.5 2.53 9.40 8.16 3.7 3.2
400 4.1 0.67 4.00 2.50 6.0 3.7
450 1.8 0.17 2.00 0.98 11.8 5.8
800 350 �/10 5.30 7.99 �/10 1.5 �/2
400 �/10 1.96 4.05 4.98 2.0 2.5
450 �/10 0.15 1.78 1.68 11.7 11.1
The gas sensitivity (S ) is defined as the ratio of the sensor resistance in air (Ra) to that in the test gas (Rg), and the selective coefficient (Kgas/gas?)
between two different gases is defined as the ratio of the gas sensitivity of one tested gas (Sgas) to that of another (Sgas?).
Y. Chen et al. / Materials Science and Engineering B99 (2003) 52�/55 55