Investigation on the structure, electrical conductivity and ethanol gas sensitive properties of...

44 International Journal of Nanomaterials and Biostructures 2012; 2(3) 44-49

ISSN 2277-3851

Original Article

Investigation on the structure, electrical conductivity and ethanol gas sensitive

properties of Ce-doped SnO2 nanoparticles sensors

M. E. M. Hassouna 2, A. M. El-Sayed

1, *, F. M. Ismail

1, M. H. Khder

2, A. A. Farghali

2, and S. M. Yakout

1

1 Inorganic Chemistry Department, National Research Centre, Tahrir Street, Dokki, 12622 Cairo, Egypt 2 Department of Chemistry, Faculty of Science, Beni-Sueif University, Egypt

*Correspondence author e-mail address: [email protected]

Received 11 July 2012; accepted 28 July 2012

Abstract Nanoparticles tin oxide gas sensors having the chemical formula SnO2 + x wt% CeO2 (x = 0, 2, 4 and 6) have been

synthesized by chemical precipitation method and sintered at 400, 600 and 800 oC. The composition and the morphology

of the prepared samples are investigated and characterized by using XRD, IR, SEM and TEM techniques. XRD and IR

results confirmed the formation of SnO2 tetragonal rutile and CeO2 cubic structures. The SEM and TEM investigations revealed that the average particle size of SnO2 increased with increasing the sintering temperature and decreased with CeO2

additions. The electrical conductivity was found to increase with sintering temperature and CeO2 additions. The obtained

gas sensing properties data explained that the sensor having 2 wt % CeO2 and sintered at 400◦C has the highest sensitivity,

rapid response time and short recovery time to ethanol gas among the prepared sensors. The influence of sintering

temperature and CeO2 content on the structure, electrical conductivity and ethanol gas sensing of SnO2 sensors is

discussed.

© 2011 Universal Research Publications. All rights reserved

Key words: SnO2 nanoparticles, Ce-doping, gas sensor, ethanol gas.

1. Introduction In recent years, the nano-structured semiconductor metal

oxides have been widely studied as possible candidates for

using in the gas sensors applications [1]. Ethanol vapour is

one of the most popular gases in industry and our daily life

[2]. The detection and quantification of ethanol gas

concentrations have wide applications in different fields

such as monitoring of fermentation, medical processes,

food industries, foodstuffs conservering and breathe analyzer of drivers to reduce the number of roads accidents

caused by excessive alcohol consumption [3-6].

Nanostructured tin oxide (SnO2) has been studied for gas-

sensing applications and is deemed to be one of the most

promising materials to generate high-performance gas-

sensing devices for ethanol gas detection [7]. The

advantages of the SnO2 sensor are the high sensitivity, low

weight and cost. The working principle of the

semiconductor tin oxide sensors for ethanol gas detection is

based on the changes in the electrical resistance produced

by the interaction of the ethanol gas with the chemisorbed

oxygen species (O− or O2−) on the surface of the sensor [4, 6]. The reaction of ethanol gas with the pre-adsorbed

surface oxygen species is accompanied with the liberation

of electrons to the conduction band of the tin oxide material

which leads to the change in the electrical resistance. The

gas sensing properties of the tin oxide can be improved by

decreasing its particles size and consequently increasing the

surface area which provides more surface active sites for

adsorption and interaction of gases [8]. The modification of

surface and bulk properties of tin oxide through doping is

an effective way to improve its gas sensing properties.

Many researches have attempted to enhance the gas sensing

properties of tin oxide towards ethanol gas through using some additives such as Pt, Pd, Au, TiO2, CdO, Fe2O3 and

La2O3 [9-15]. Rare earth oxides additions are well known

to display a high surface basicity, fast oxygen ion mobility

and interesting catalytic properties which are very

important in gas sensing [16]. The present study aims to

prepare SnO2 nanoparticles and study the effect of sintering

temperature and CeO2 doping concentrations on the

structural, electrical conductivity and gas sensing properties

of the prepared SnO2 nanoparticles toward ethanol gas.

2. Experimental

Tin oxide nanoparticles were prepared by chemical

precipitation method using tin tetrachloride (99.7 %) and diluted NH4OH as starting materials. An aqueous solution

of tin tetrachloride 0.2 mol/L was prepared in distilled

water. Diluted NH4OH solution was added dropwise to the

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Fig. 1: X-ray diffraction patterns of the prepared SnO2 + x

wt% CeO2 samples sintered at (a): 400 oC and (b): 800 oC.

prepared tin tetrachloride solution with continuous stirring

until the pH reached 9, whereupon white gelable precipitate

of Sn(OH)4 was obtained. The precipitate was filtered and

washed thoroughly until free of chloride by testing the

filtrate with silver nitrate solution. The obtained precipitate

was dried in air and ground to a fine powder followed by

calcination at 400 oC for 4 hours then left to cool in

atmospheric air to room temperature. Proper amounts of

CeO2 were added to the prepared SnO2 powders with ratio

of 0, 2, 4 and 6 wt % and then ground in an agate mortar to

get homogeneous and good mixing mixtures. The resulting mixtures were used as a functional material to fabricate

sensor pellets of 12 mm in diameter and 2 mm thick,

followed by sintering at 400, 600 and 800 oC for 2 hours.

The prepared samples were investigated using X-ray

powder diffraction (XRD) by X-ray diffractometer (model-

Bruker AXS D8 advance) with copper radiation, infrared

spectra (IR) by Nexus 670 FTIR spectrophotometer

(Nicolet, USA). Scanning electron microscope (SEM) by

SEM JEOL models, JXA-480A Electron Probe Micro

analyzer. Transmission electron microscope (TEM) by

JEOL JEM-1230 operating at 120 KV attached to CCD camera. a. c. electrical conductivity as well as ethanol gas

sensing properties of the different sensor samples were

measured in the temperature range from 30 up to 410 oC

using LCR meter (Hitester, model Hioki 3532, made in

Japan) at frequency of 1 KHz and applied 5 V. The sample

chamber is a closed glass cell contains the sensor sample

surrounded by electrical cylindrical ceramic furnace with

controlled temperature device. The desired ethanol gas

concentrations were obtained by injection a known volume

of ethanol vapor using a micro-syringe through air tight

rubber port into the glass chamber. The sensitivity (S) of

the sensor samples were measured and calculated as the ratio of the electrical resistance in air atmosphere (Rair) to

that in air containing ethanol gas (Rgas) in the temperature

range from 30 up to 410 oC by using the following relation

[13]:

S = Rair / Rgas (1)

3. Results and discussion

3.1. Characterization

The X-ray diffraction investigation carried out on the

Fig. 2: Infrared spectra of the prepared SnO2 + x wt% CeO2

samples sintered at (a): 400 oC and (b): 800 oC.

prepared SnO2 revealed that the obtained SnO2 is pure single phase having tetragonal rutile structure with a good

crystalline character according to the standard data

(JCPDS, card file No. 41-1445). The average crystallite

size (D) of the prepared SnO2 is approximately 7.2 nm as

estimated from the full width at half maximum (FWHM) of

the most intense peaks using Debye-Scherer equation

which confirmed the formation of nanoparticles SnO2.

Figure 1 depicts the XRD patterns of the pure and CeO2-

doped SnO2 samples sintered at 400 and 800 oC. All the

diffraction peaks in the patterns can be indexed to the tetragonal rutile structure of SnO2 and to the cubic CeO2

structure (JCPDS No. 81-0792). Diffraction peaks due to

CeO2 began to appear with the concentration of 2 wt % and

increased in number and intensity with CeO2 concentration.

Where, the sample having 2 wt% CeO2, only two small

peaks related to CeO2 located at 2θ = 28.5 and 47.4 are

observed. By increasing the CeO2 content up to 6 wt %

further peaks of CeO2 situated at 2θ =33.1 and 56.3

appeared. These results suggest that there is no chemical

reaction occurred between SnO2 and CeO2 components

during the sintering processes. The average crystallite size of the pure and CeO2-doped SnO2 samples sintered at 400,

600 and 800 oC are estimated and tabulated in Table 1. It

was found that, the average crystallite size of SnO2

increased with sintering temperature and decreased with

CeO2 content.

Figure 2 shows the infrared spectra of pure and CeO2-

doped SnO2 samples sintered at 400 and 800 oC. The band

appeared at the range of 615-619 cm-1 (ν1) is attributed to

Sn-O-Sn stretching vibration while that appeared at about

540 cm-1 is attributed to Sn-O terminal oxygen vibration

[17]. The obtained bands position (ν1) is tabulated in Table

1. For samples with 4 and 6 wt % CeO2, small bands are appeared nearly at 433 cm-1 which are assigned to CeO2

doping material.

The scanning electron microscope photographs of the pure

and CeO2-doped SnO2 samples sintered at 400 and 800 oC

are shown Figures 3 and 4 respectively. It was found that,

the grains size of the pure and CeO2-doped SnO2 samples

tends to increase with sintering temperature. This may be

due to the promotion of crystalline phase in the powder and


Fig.3: SEM photographs of SnO2 + x wt% CeO2 samples sintered at 400

oC, (a): x = 0, (b): x = 2, (c): x = 4 and (d): x =

6.

neck growth between particles as the temperature increased

[18]. The SnO2 grains size was found to decrease with

increasing CeO2 content at the same sintering temperature

which indicates that the addition of CeO2 may be restricting

the grains growth of SnO2.

Fig. 4: SEM photographs of SnO2 + x wt% CeO2 samples

sintered at 800 oC, (a): x = 0, (b): x = 2, (c): x = 4 and (d): x

= 6.

Fig. 5: TEM photographs of SnO2 + x wt% CeO2 samples


= 6.

Fig. 6: TEM photographs of SnO2 + x wt% CeO2 samples


= 6.

Figures 5 and 6 depicts the transmission electron

microscope photographs of the pure and CeO2-doped SnO2

samples sintered at 400 and 800 oC respectively. The

Photographs of the samples sintered at 400 oC, Fig. 5, show

formation of nano-sized particles having nearly spherical

shape with average particle size ranging from 11.5 to 22.2 nm. With increasing the sintering temperature up to 800 oC,

Fig 6, the average particle size of the samples is found to

increase (ranging from 24.2 to 40.4 nm). Also, the average

particle size of the pure SnO2 was found to decreases with

CeO2 additions at the same sintering temperature which

confirms that the addition of CeO2 inhibits the growth of

SnO2 particles.

3.2. Electrical conductivity study

The variation of the electrical conductivity with

temperature for pure and CeO2-doped SnO2 samples

sintered at 400 and 800 oC is shown in Figure 7. Generally,

the features of the curves are nearly similar. Each curve consists of three different temperature regions denoted as

AB, BC and CD and separated by knees. At low

temperature regions (AB), the electrical conductivity

slightly increases with temperature which may be attributed

to the thermal activation of the charge carriers to the

conduction band of SnO2 [19]. The decreases in the

electrical conductivity in BC regions are likely to be in

relation with the adsorbed of oxygen species on the

surfaces of the SnO2 nanoparticles [20]. The adsorbed

oxygen molecules turning into oxygen ions

(O2−→2O−→O2−) by accepting free electrons from the

SnO2 conduction band which leads to the increase in the

electrical resistance and consequently the decreasing in the

electrical conductivity. With increasing the temperature, the

intrinsic behaviour is dominant and the electrical

conductivity in the CD regions increased again, which

probably due to the thermal excitation of electrons and

desorption of oxygen species.

With increasing the sintering temperature the electrical

conductivity increased (Fig. 7) which may be attributed to

the increase of the charge carriers' concentration due to the

increase of oxygen vacancies by increasing the sintering


Fig. 7: The variation of the electrical conductivity with

temperature for SnO2 + x wt% CeO2 samples sintered at

(a): 400 oC and (b): 800 oC.

temperature [18]. Also, with rising the sintering temperature the boundaries of the grains get diffused which

resulting in the increase in the grains size which causes the

electrical conductivity increases. Also, the electrical

conductivity was found to increase with increasing CeO2

content at different sintering temperature, Fig. 7. This may

be due to the increase in the charge carriers' concentration

[21]. The intrinsic activation energy (∆E) of the measured

samples at higher temperature region was calculated and

presented in Table 1.

Table 1: The obtained data of SnO2 + x wt % CeO2 samples sintered at different temperature.

Fig. 8: The variation of the sensitivity to ethanol gas with

temperature for SnO2 + x wt % CeO2 sensor samples

sintered at (a): 400, (b): 600 and (c): 800 oC.

3.3. Gas sensing properties

Figure 8 shows the variation of the sensitivity to ethanol

gas with temperature for pure and CeO2-doped SnO2 sensor

samples sintered at 400, 600 and 800 oC. It can be seen

that, the sensitivity gradually increases with increasing the

temperature and attains the maximum values at 300 oC for all sensor samples. With increasing the temperature above

300 oC, the sensitivity of the sensor samples decreased. The

high sensitivity of the sensor samples to ethanol gas at 300 oC can be explained depending on the gases adsorption

model where, the sensitivity of the sensor samples depends

on the interaction between the adsorbed oxygen species and

ethanol gas on their surfaces [4, 6]. With rising the

temperature of the sensor samples above 150 oC, the

adsorbed oxygen species concentration (O2−→2O−→O2−)

gradually increase until certain temperature by accepting

electrons from the conduction band which accompanied

with the increase in the electrical resistance. So, the sensitivity attained the maximum values at 300 oC due to

the high concentration of the chemisorbed oxygen species

and the high oxidation activity of ethanol gas and the

reaction can be written as follow:

At higher temperature (> 300 oC), desorption of the oxygen

species from the sensor surfaces occurred which leads to

the decrease of oxygen species concentration and

consequently the sensitivity decreased.

The sensitivity to ethanol gas of the measured sensor

samples decreased with increasing the sintering

temperature from 400 up to 800 oC as shown in Fig.8. The

decreasing in the sensitivity in this case may be due to the

effect of the changing in the particle size [8]. Where, the sensor samples with small particles having high surface

area which provide more active sites at which the gaseous

species adsorb and interact. As the average particle size of

the sensor samples increased with sintering temperature

from 400 to 800 oC, the surface area of these sensors

consequently decreased which reflected in the decrease in

the sensitivity with sintering temperature.

Fig. 9: The variation of the sensitivity with CeO2 content

for SnO2 + x wt % CeO2 sensor samples sintered at

different temperature, operating temperature 300 oC.


Fig. 10: The variation of sensitivity with ethanol gas

concentration for SnO2 + x wt % CeO2 sensor samples

sintered at 400 oC, operating temperature 300 oC.

The variation of the sensitivity to ethanol gas with CeO2

content for SnO2 + x wt % CeO2 sensor samples sintered at

different temperature is shown in Figure 9. It can be seen

that, the highest sensitivity values obtained with the

addition of 2 wt % CeO2 while the sensitivity decreased with further additions at different sintering temperature.

The additions of 2 wt % CeO2 to SnO2 seem to be benefit

for ethanol dehydrogenation on the surface of the sensor

sample. Above 2 wt %, CeO2 content the available

adsorption sites on the SnO2 sensor surface may be reduced

which worsen the gas-sensing properties. Therefore, the

appropriate CeO2 doping concentration seems to be

important to improve the gas sensing properties.

Figure 10 shows the variation of the sensitivity with

ethanol gas concentration for pure and CeO2-doped SnO2

sensor samples sintered at 400, operating temperature 300 oC. For the undoped SnO2 sensor, the sensitivity linearly increased with increasing the concentration up to 300 ppm

then after it slowly increased with gas concentration until

2000 ppm. While, the doped sensors show linear increasing

in sensitivity up to 500 ppm then after it slowly increased

with increasing the gas concentration.

Fig. 11: The variation of the sensitivity with time (seconds)

for undoped SnO2 (●) and SnO2 containing 2 wt% CeO2

(∆) sensor samples sintered at 400 oC during (a) exposure

to 100 ppm ethanol gas and (b) re-exposure to air,

operating temperature 300 oC.

The response time for the sensor is usually defined as the

time taken to achieve at least 90 % of the final change in its

electrical resistance during gas exposure. While, the

recovery time is generally defined as the time taken for the

sensor to get back at least 90 % of its original state when

re-exposure to air ambient by maintaining the operating temperature constant [1, 6]. Figure 11 shows the variation

of the sensitivity with time (seconds) for pure and SnO2 + 2

wt% CeO2 sensor samples sintered at 400 oC during

exposure to 100 ppm ethanol gas and re-exposure to air

respectively. With respect to the pure SnO2 sensor sample,

the response time and recovery time were 25 and 20 second

respectively. While, the sensor sample containing 2 wt%

CeO2, the response time and recovery time were 20 and 15

second respectively. From the above mentioned results it

can be concluded that SnO2 + 2 wt% CeO2 sensor sample

sintered at 400 oC is the more suitable to be used as ethanol

gas sensor where it has the highest sensitivity value, rapid response time and short recovery time.

4. Conclusion CeO2-doped SnO2 nanoparticles sensors were synthesized

by chemical precipitation method. XRD and IR

investigations confirmed the formation of SnO2 tetragonal

rutile and CeO2 cubic structures. The SEM and TEM

photographs revealed that the average particle size of SnO2

increased with increasing the sintering temperature and

decreased with CeO2 additions. The electrical conductivity

was found to increase with sintering temperature and CeO2

content. Ethanol gas sensing properties of SnO2 sensors were enhanced with CeO2 doping content. Among the

prepared SnO2 nanoparticles sensors it was found that the

sensor containing 2 wt % CeO2 and sintered at 400 oC has

the highest sensitivity to ethanol gas, rapid response time

and short recovery time.

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Source of support: Nil; Conflict of interest: None declared

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