electrical and gas - sensing properties of wo3 semiconductor material

The e€ects of calcining temperature and operating temperature on electrical resistance and sensitivity, and sensitivity-gas concentration properties of the WO3-based sensors were investi

Electrical and gas-sensing properties of WO 3

semiconductor material

Department of Materials Science and Engineering, Yunnan University, Kunming 650091, People's Republic of China Received 28 November 2000; received in revised form 22 January 2001; accepted 19 February 2001

Abstract

In this paper, the electrical and gas-sensing properties of calcined tungsten trioxide semiconductor materials were investigated X-ray di€raction, scan electron microscopy and infrared were used to characterize structure and perfor-mance of WO3semiconductor material The average grain size of WO3was 22 nm after calcination at less than 800°C and 24±26 nm at more than 900°C for 1 h The sensors of indirect heating type were fabricated The e€ects of calcining temperature and operating temperature on electrical resistance and sensitivity, and sensitivity-gas concentration properties of the WO3-based sensors were investigated The sensor based on WO3 exhibited high sensitivity and good response characteristics to ethanol gas The electrical properties of WO3were analyzed and the sensitive mechanism was discussed Ó 2001 Elsevier Science Ltd All rights reserved

Keywords: WO 3 ; Gas-sensing properties; Calcination temperature; Operating temperature; Sensitivity

1 Introduction

Numerous metal oxide semiconductor materials were

reported to be usable as semiconductor gas sensors, such

as ZnO, SnO2, WO3, TiO2, a-Fe2O3 and so on These

candidates have non-stoichiometrics structures, so free

electrons originating from oxygen vacancies contribute

to electronic conductivity [1] The demands for accurate

and dedicated sensors to provide precise process control

and automation in manufacturing process, and also to

monitor and control environmental pollution, have

ac-celerated the development of new sensing materials and

sensors technology over the last decade [2,3] Some new

types of sensing materials are still being studied and

exploited at present time WO3 is n-type semiconductor whose electron concentration is determined mainly by the concentration of stoichiometric defects such as oxy-gen vacancy like other metal oxide semiconductors [4]

WO3gas sensor was ®rst reported for detection of H2by

®lms changed greatly upon the exposure to the H2 am-bient Following this pioneering work, many works have been performed on the structural, electrical properties and sensing characteristics of WO3 ®lms It was dem-onstrated by di€erent authors that WO3-based thin and thick ®lms were both sensitive to NOxgas [1,4,6±10] It has been reported that WO3materials have good sensi-tivity for low concentration of NOx gas [6] However, most reports focused on the NOx gas sensors, and the study of sensing to other gases was rare In this paper,

we have investigated the electrical and gas-sensing characteristics of WO3 semiconductor material to other gases such as ethanol, petrol, butane and methane The experimental results indicated that the sensor based on

characteristics to ethanol gas The electrical and gas-sensing mechanism were analyzed and discussed

* Corresponding author Present address: Department of

Materials Science and Engineering, Tsinghua University,

Beij-ing 100084, People's Republic of China Tel.:

+86-0871-5033371; fax: +86-0871-5031410.

E-mail address: wang_yude@263.net (W Yu-De).

0038-1101/01/$ - see front matter Ó 2001 Elsevier Science Ltd All rights reserved.

PII: S0038-1101(01)00126-5

2 Experimental

2.1 Materials

All the chemical reagents used in the experiments

were purchased from commercial sources as

guaranteed-grade reagents and used without further puri®cation

2.2 Characterization of samples

The crystal structures of WO3samples were analyzed

using X-ray di€raction (XRD, Rigaku D/MAX-3B

powder di€ractometer) In order to obtain high

resolu-tion and to minimize the signal to noise ratio, we have

performed the measurements with ®xed slits The mean

crystallite sizes (D) were measured from XRD peaks that

were obtained at a scan rate of 2° min 1 D is based on

the Scherrer's equation: D ˆ k=…DW cos h† Where k is

the wavelength of X-ray (k ˆ 1:5418 A), h the Bragg's

di€raction angle, and DW the true half-peak width The

microstructure of powder was characterized by scan

electron microscopy (SEM, CSM950) The conductance

type of the WO3was measured with the hot probe

2.3 Fabrication of sensor elements

In order to prepare series of sensors, we have chosen

the indirect heating type as the structure of sensor The

sensor were fabricated according to the literature [11]

WO3 semiconductor materials with SiO2 (4 wt.%) were

fabricated on an alumina tube with Au electrodes and

Platinum wires The SiO2 was used to add intensity of

sensitive material of WO3, but it did not in¯uence the

gas-sensing properties of WO3-based sensor A Ni±Cd

alloy crossing alumina tube was used as a resistor This

resistor ensured both substrate heating and temperature

control Each element was sintered at di€erent

temper-ature (350±800°C) for 1 h in air Thickness of the

sen-sitive bodies after sintering was approximately 0.6±0.8

mm

2.4 Measurement of sensing properties

The gas-sensing properties were examined in a

chamber through which air or a sample gas (petrol,

methane, butane and ethanol diluted with air) was

al-lowed to ¯ow at a rate of 160 cm3/min The sensor's

resistance was measured by using a conventional circuit

in which the element was connected with an external

resistor in series at a circuit voltage of 10 V The

elec-trical response of the sensors was measured with an

automatic test system, controlled by a personal

com-puter In order to improve their stability and

repeat-ability, the gas sensors were aged at 250°C for 150 h in air The gas sensitivity (b) was de®ned as the ratio of the electrical resistance in air (Ra) to that in gas (Rg)

3 Results 3.1 Structure of samples The average grain size of WO3 was 22 nm after cal-cination at less than 800°C and 24±26 nm at more than 900°C for 1 h The X-ray powder di€raction patterns of

shown high degree of crystallinity (Fig 1) Their particle sizes based on Scherrer's equation are 21 nm The av-erage particle sizes of SEM show consistency with the results of XRD With the increasing of calcining

peaks, the size of grains and macro pores gradually in-creased According to the examined results by the hot probe, WO3 is n type semiconductive material This re-sult is in good accordance with the literature [4] 3.2 Resistance±temperature characteristics Fig 2 shows resistance±calcination temperature curves for the WO3-based sensor in the air The resis-tance of sensor appeared the largest value for low calc-ining temperature and the smallest value for high calcining temperature at the operating temperature of 200°C and 250°C, respectively As shown in Fig 3, the resistance±operating temperature properties of the sen-sor shows the characteristic of a typical surface-con-trolled model [12±14] Electron concentration of WO3

semiconductor is determined mainly by the concentra-tion of stoichiometric defects such as oxygen vacancy like other metal oxide semiconductors From 100°C to

Fig 1 X-ray powder di€raction pattern of WO 3 calcined at 500°C for 0.5 h (a) monoclinic WO ( b) triclinic WO

640 W Yu-De et al / Solid-State Electronics 45 (2001) 639±644

300°C, the shape of their resistance±operating

temper-ature curves is attributed to the change in charge state of

the chemisorbed oxygen-related species, such as O2ads,

Oads, OHads and O2

ads However, above 300°C their in-trinsic defects, such as oxygen vacancies are responsible

for the conductance of the sensor Generally, a higher

sintering temperature is needed during the fabrication of

gas-sensing elements, and gas sensors have to operate in

the temperature range from 200°C to 400°C for a long

time [15] So it is important for gas sensor to have good

thermal stability In Fig 3, it can be seen that the e€ect

of operating temperature from 175°C to 225°C on

re-sistance of WO3sensor is smaller than that of the other

temperatures So that sensor's resistance changed little

in this temperature range, and the sensor based on WO3

has good thermal stability when their operating

tem-peratures are in this range This thermal stability is of

signi®cance to apply the sensors to certain control and

monitoring

3.3 Gas-sensing properties

In this study, we ®rst examined the e€ect of calcining temperature on gas-sensing properties of sensor It was found that the calcining temperature has great in¯uence

on the sensitivity of the sensor to the sample gases, such

as ethanol, petrol, and butane Fig 4 shows the gas sensitivity to the sample gases changes as a function of calcination temperature for WO3-based sensors at 200°C operating temperature It can be clearly realized that there are an increase in sensitivity for ethanol (100 ppm) gas as the calcining temperature increasing from 350°C

to 500°C, and the sensitivity decreased above 550°C It is seen from the above results that as the calcination temperature increase, the crystallite size increases, thereby decreasing the surface area, which in turn af-fected the gas sensitivity In order to maintain the crystallite size, WO3 sensor calcined at 500°C has been chosen, since this is the temperature at which the max-imum sensitivity is obtained for ethanol gas

It is clear from Fig 5 that the operating temperature has an obvious in¯uence on the sensitivity of sensor to

Fig 2 The resistance±calcination temperature behavior of the

WO 3 -based sensor in air at operating temperature (a) 200°C

and (b) 250°C.

Fig 3 The relation between the resistance and operating

temperature in ambient humidity air.

Fig 4 The in¯uence of calcinations temperature on the sensi-tivity …gas concentration ˆ 100 ppm).

Fig 5 The in¯uence of operating temperature on the sensi-tivity of the sensor for ethanol and petrol 100 ppm, and for butane and methane 1000 ppm.

sample gases The maximum sensitivity to 100 ppm

ethanol gas occur at 80°C is about 40 However,

sensi-tivities to ethanol is reduced with the increasing of

op-erating temperature in the range of 80±200°C On the

other hand, the sensitivity of sensor to other gases such

as petrol, butane and methane is very low, though gas

concentration is 1000 ppm

Fig 6 shows the relationship between sensitivity and

sample gases concentration for sensor operating at

200°C (because sensor has good thermal stability at this

temperature) When the sensor is operated at 200°C, the

sensitivity exhibits a good dependence on ethanol gas

concentration Long time stability of the WO3sensor in

the whole investigated time rang is shown in Fig 7

4 Discussion

4.1 Electrical properties

The oxygen adsorbed on the surface of the material

The oxygen adsorbed depends on the particle size, large

speci®c area of the material, and the operating temper-ature of the sensor With increasing tempertemper-ature in air,

undergoes the following reactions:

O2gas$ O2ads

O2ads‡ e $ O2ads

O2ads‡ e $ 2Oads

Oads‡ e $ O2

ads

The oxygen species capture electrons from the ma-terial, leading to increasing of the hole concentration and decreasing of the electron concentration WO3 is a kind of the acidic oxide and can react with the alkali Besides the state of oxygen adsorbed on the surface of

complicated The reaction can be summarized as

Wlat‡ H2O $ …Wlat OH † ‡ H‡

ads

where Wlatis Lewis acid site, which can form covalent

which is Bronsted acid site that is can be removed easily

in catalytic reaction OH , O2adsand Oadsare dominat-ing oxygen-related species on materials surface at low temperature [16,17] As shown in Fig 3, from room temperature to 175°C, the resistance decreases with in-creasing of the operating temperature because the ther-mal energy causes electrons to emit from low energy levels (such as donor levels or valence band) to con-duction band In the range of 175±225°C, the change in resistance is very small It is because the electrons of donor level are ionized completely, and the electronic concentration of intrinsic exciting is less than the con-centration of donor in this temperature region How-ever, when the temperature is higher than 250°C,

O2 ads The resistance starts to go up in the temperature ranging from 250°C to 300°C, which may be attributed

to such electron depletive type mechanisms [15] 4.2 Gas-sensing mechanism

The gas-sensing mechanism is based on the changes

in the conductance of WO3 The reducing gas reacted with oxygen adsorbed on the surface of the sensor and the possibility of the reaction between reducing gas and lattice oxygen was very small The reducing gas acting

on the WO3 sensor surface can be explained as [18]:

To maintain neutrality, the electrons release back WO3

material, resulting in the increase of the electron

con-Fig 6 The e€ects of gas concentration on the sensitivity of

sensor at 200°C.

Fig 7 Long time stability of the sensitivity for ethanol

…gas concentration ˆ 100 ppm).

642 W Yu-De et al / Solid-State Electronics 45 (2001) 639±644

centration and the decrease of the resistance This

change of the electrical resistance determined sensitivity

of the WO3based sensor to reducing gases The reacting

studied by infrared (IR, Mattson ALPHA CENTAURT

FT-IR) spectrum As shown in Fig 8, it is found that

peaks of aether vanished at 400°C Ethylene is produced

at 350°C and the amount of it increases with

tempera-ture Ethanol is adsorbed and reacted with Lewis acid

site of surface (that is W ion) to produce O±CH2±CH3

The possible process of the reaction can be explained as

follows:

…ads† is activated

2H‡ ads‡ Oads! H2Ogas

H‡ ads‡ OHads! H2Ogas

H‡ ads‡ O2 lat ! Olat±H ‡ VO

through dehydration and the reaction product of eth-ylene increases with temperature The reaction can be written as

If temperature is low and oxygen is de®cient, two O±CH2±CH3can interact to produce ethyl ether:

Fig 8 IR spectrum of ethanol after reacting with WO 3 material at (a) 145°C, (b)200°C, (c) 250°C, (d) 295°C, (e) 350°C, (f) 407°C, (g) 455°C, (h) 500°C.

All these reactions release electrons into the WO3

ma-terial, leading to the increase of the electron

concentra-tion, and the decrease of the resistance of WO3-based

sensor This result is in good accordance with the above

analysis

5 Conclusions

used as a gas-sensing material for ethanol gas The

analysis of the electrical properties and gas-sensing

calcining and operating temperature of sensor obviously

in¯uence on the resistance change and gas sensitive

characteristics of the WO3 sensor At calcining

temper-ature of 500°C and operating tempertemper-ature of 200°C, the

gas sensors based on WO3 has good thermal stability,

sensitivity and response characteristics to ethanol gases

Acknowledgements

This work was supported by the Natural Science

Foundation of Yunnan Province, China

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