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.

References

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[3] G. Zhang, M. Liu, Sens. Actuators B 69 (2000) 144.

[4] S.H. Park, Y.C. Son, W.S. Willis, S.L. Suib, K.E. Creasy, Chem.

Mater. 10 (1998) 2389.

[5] A.R. Phani, Appl. Phys. Lett. 71 (16) (1997) 2358.

[6] C. Bulpitt, S.C. Tsang, Sens. Actuators B 69 (2000) 100.

[7] C. Nayral, T. Ould-Ely, A. Maisonner, B. Chaudret, P. Fau, L.

Lescouzes, A. Peyre-Lavigne, Adv. Mater. 11 (1999) 61.

[8] S.N. Frank, A.J. Bard, J. Phys. Chem. 81 (1977) 1484.

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