improvement of long-term stability in sno2-based gas sensor for monitoring offensive odor

Ž .Sensors and Actuators B 56 1999 50–58

Improvement of long-term stability in SnO -based gas sensor for2

monitoring offensive odor

Jong Hyun Park, Kwang Ho Kim )

Department of Inorganic Materials Engineering, Pusan National UniÕersity, Pusan 609-735, South Korea

Received 20 March 1998; received in revised form 14 December 1998; accepted 15 December 1998

Abstract

WO rSnO ceramics has been suggested as an effective sensing material for monitoring offensive odor or pollutant gases. This work3 2

was focused on improving long-term stability, which has been a principal problem generally associated with SnO semiconductor gas2

sensors. Miniaturized thick film gas sensors were fabricated by screen-printing technique. Two types of sensor materials, W-doped SnO2

and WO -mixed SnO , were comparatively investigated with respect to long-term stability and sensitivity to several gases. Small amount3 2Ž . Ž .of W doping 0.1 mol% into SnO largely improved the long-term stability. The W 0.1 mol% -doped SnO gas sensor had higher2 2

Ž . Ž .sensitivity to both acetone and alcohol compared with WO 5 wt.% -mixed SnO gas sensor. On the contrary, WO 5 wt.% -mixed3 2 3

SnO gas sensor showed superior sensitivity to cigarette smoke due to larger W content. q 1999 Elsevier Science S.A. All rights2

reserved.

Keywords: Odor sensor; WO rSnO ceramics; Long-term stability; W-doping3 2

1. Introduction

Semiconductor gas sensors are widely used for applica-tion in gas sensing. They offer many advantages such assimple fabrication, low cost, high sensitivity to both reduc-ing and oxidizing gases, etc.

Odor sensing is one of the new trends in the applicationfields of semiconductor gas sensors because of the protec-tion of the environment, the atmosphere in the work placeand health science. Among the raw materials of gas sen-sors, SnO is the typical sensing material for air pollutant2

Ž .gases like formaldehyde, trimethylamine TMA , butylw xacid, ethanol, etc. 1–5 . On the other hand, WO shows3

especially high sensitivity to H S gas which has typical2w xoffensive odor 6,7 . Therefore, the ceramic mixture con-

Ž .sisting of both SnO and WO 5 wt.% has been sug-2 3

) Corresponding author

gested as an effective sensing material for monitoringw xair-pollutant gases 8 .

It was, however, found that the gas sensor using theceramic mixture of SnO rWO had a problem concerning2 3

long-term stability. In actual use, the resistance of thesensor steadily increased with operation time and finallythe sensor was impossible to operate. The increase ofresistance of the SnO sensing film is considered to be2

attributed to its conduction mechanism of nonstoichiome-try because the nonstoichiometric oxides generally reactwith ambient oxygen. For the improvement of the long-termstability of SnO -based sensing films, it is considered to2

replace the nonstoichiometric conduction mechanism withanother conduction mechanism, for example, the con-trolled valence mechanism.

In this paper, W-doped SnO powder, in which Sn sites2

in the SnO crystal lattice were substituted for W ions, was2

prepared by coprecipitation. The long-term stability andsensitivity to pollutant gases were comparatively investi-gated between W-doped SnO gas sensor and WO -mixed2 3

SnO gas sensor.2

0925-4005r99r$ - see front matter q 1999 Elsevier Science S.A. All rights reserved.Ž .PII: S0925-4005 99 00065-9

( )J.H. Park, K.H. KimrSensors and Actuators B 56 1999 50–58 51

Fig. 1. Flow chart of manufacturing process of W-doped SnO powder.2

2. Experimental

2.1. Preparation of sensing materials

To prepare the W-doped SnO powders for the gas2

sensor, the coprecipitation method was used. Fig. 1 showsthe manufacturing process of W-doped SnO powders.2

WCl was dissolved in distilled water heated at 808C, then6

SnCl was added to the aqueous solution. The SnCl and4 4

WCl aqueous solution was neutralized with NH OH to6 4

obtain coprecipitate of W-doped tin oxide hydrate. Thecoprecipitate was then thoroughly washed with distilledwater to remove residual ammonium and chlorine ions,and finally it was dried and calcined in air. Fig. 2 showsXRD patterns of the precipitate calcined at various temper-atures ranging from 5008 to 8008C for 3 h. The as-receivedprecipitate was amorphous. Diffraction peaks correspond-

Fig. 2. XRD patterns of precipitate with calcination temperature from 5008 to 8008C.

( )J.H. Park, K.H. KimrSensors and Actuators B 56 1999 50–5852

ing to polycrystalline SnO , however, appeared after calci-2

nation of the precipitate at temperatures above 5008C. Theprecipitate was well crystallized with increase of calcina-tion temperatures, and diffraction peaks of the precipitatecalcined at 8008C was fitted exactly with those of commer-cial SnO powder.2

ŽOn the other hand, the ceramic mixture WO 53. w xwt.% rSnO , which was suggested by Yun et al. 8 , was2

prepared using commercial WO and SnO powders3 2Ž .99.99%, Aldrich for the comparative study with ourcoprecipitated powder. The average grain sizes of ourcoprecipitated powder and commercial SnO powder were2

around 20 nm and 40 nm, respectively, through calculationfrom XRD data using the Scherrer equation.

2.2. Fabrication of sensor

Fig. 3 shows the schematic view of our thick film gassensor consisting of a sensing layer, electrodes, and heaterformed on Al O substrate by a screen-printing technique.2 3

The sensor fabrication procedure was as follows:Ž .i formation of Pt patterns on alumina substrate of

2 Ž3.5=3.5 mm for electrodes and heater fired at 11008C.for 1 h in air ;

Ž .ii paste preparation by thoroughly mixing the sensingŽ .materials powders with an organic binder;

Ž .iii printing of sensing layer in thickness of about 5mm.The printed sensing layer was heated at 1508C for 30

min to eliminate organic binder, sintered at 6008C for 1 hto improve the adhesion between the SnO particles and2

Al O substrate, and then fired at 8008C for 30 min along2 3

Pt wire bonding. The fabricated sensing layer was 1.0=1.0mm2 in size and the distance of two electrodes was 0.2mm. Fig. 4 shows scanning electron microscope micro-graphs of the fabricated Pt electrode, sensing layer, andinterface of electrodersensing layer. Metal Pt is generallyw x9,10 used as the electrode for ceramic materials becauseit improves the adhesion of electroderceramics and en-ables the interface to be ohmic contact. The sintered Ptelectrode had very rough structure with large pores in ourwork. In addition, the sensing layer consisting of relativelyfine particles was spread into the skeleton of the Pt elec-trode. This kind of binding between metal electrode andceramic layer largely improved the mechanical strength ofthe interface due to three-dimensional interlocking. Sincemany interfacial contacts between Pt electrode and sensinglayer were formed, it was expected that contact resistance

Fig. 3. Schematic diagram of thick film gas sensor.


Ž . Ž . Ž .Fig. 4. SEM micrographs of a Pt electrode, b sensing layer, and cinterface of electrodersensing layer.

Table 1Resistance of SnO sensing layer with W concentration2

W concentration Resistance of sensing filmŽ .at room temperature

Ž .5 mol% 7.5 wt.% doped SnO 1–10 kV2Ž .1 mol% 1.5 wt.% doped SnO 100–1000 kV2Ž .0.5 mol% 0.8 wt.% doped SnO 10–20 MV2Ž .0.1 mol% 0.15 wt.% doped SnO 430 MV2

Ž .WO 5 wt.% -mixed SnO 430 MV3 2

of electrodersensing layer, if any, was diminished enoughto ignore. On the other hand, the sensing layer had manyfine pores. This desirable porous structure enlarges thesurface area of sensing layer, which means increase ofadsorption sites. Since the sensing mechanism of gas sen-sors is based on gas adsorption on the ceramic surface, theincrease of adsorption sites is important to improve thesensitivity to pollutant gases. The thick film gas sensorshown in Fig. 3 was packed with plastic capsule after Ptwire bonding.

Table 1 shows the variation of resistance of W-dopedSnO sensing layer with W concentration. The resistance2

of the fabricated sensing layer as described above wasŽ .measured by a digital tester HIOKI, 3231 . The resistance

of SnO sensing layer with small W doping below 0.12

mol% showed very high resistance at room temperature,while, the W-doped SnO sensing layer with W content2

over 0.1 mol% showed relatively low resistance. Thedecrease of resistance with increase of W doping concen-tration was attributed to increase of carrier electron, be-

Fig. 5. Schematic diagram of measurement circuit.


Ž .cause W q6 behaved as a donor in SnO crystal. Since2

base resistance of sensing layer drastically decreases at theoperation temperature of 3008C, it is hard to measure thechange of the electrical resistance of the sensing layer

Župon exposure to pollutant gases. In this reason, W 0.1.mol% -doped SnO sensing layer was selected as a sample2

Ž .for the comparative study with WO 5 wt.% -mixed SnO3 2w xsuggested by Yun et al. 8 .

2.3. Measurement of long-term stability and sensitiÕity

Fig. 5 shows a schema of the measurement circuit. TheŽ .input circuit voltage V sdc 5V was constantly appliedin

Ž . Žacross R resistance of sensor element and R resis-s L.tance of load resistor, 10 kV . V voltage across the R ,out L

which is dependent on the change of R , was continuouslys

recorded during the operating time. To compare long-termstability between WO -mixed SnO and W-doped SnO3 2 2

gas sensors, two sensors were aged at 3508C in dry air for3 days in advance, and then were heated to 3008C andmaintained at this temperature in ambient air during thewhole operating time. In order to compare relative sensitiv-ities of the two types of sensors they were exposed toalcohol, acetone and cigarette smoke. The sensor elementwas set in a measurement chamber which had an internalvolume of 24.36 l. By the method in which liquid dropswere evaporated on the small hot plate located in thechamber, the gas concentrations of acetone and alcohol inthe chamber were approximately adjusted to 300 ppm and

Ž .500, respectively. The gas sensitivity DV was defined asV yV , where V and V are the voltageout,gas out,air out,gas out,air

upon test gas and air, respectively.

Fig. 6. Variation of the V voltage of two types of sensors with operating time.out


3. Results and discussion

3.1. Long-term stability

Fig. 6 shows the variations of the V voltage of twoout

types of sensors with operating time at 3008C in ambientair. The two sensors were successively powered and heatedto be maintained at 3008C during the whole operating time.The initial V voltage of W-doped SnO and WO -mixedout 2 3

SnO gas sensor were 2.2 and 2.5 V, respectively. Whereas2

the V of WO -mixed SnO gas sensor was graduallyout 3 2

decreased with operating time, that of the W-doped SnO2

gas sensor was maintained with operating time. This resultsuggests that two types of sensors had different electrical

w xconduction mechanisms 11,12 . The electrical conductionof WO -mixed SnO was dominated mainly by the electri-3 2

cal property of SnO because SnO was matrix phase.2 2

SnO had the nonstoichiometric composition generally2

associated with oxygen vacancies. The carrier electronswere produced due to the oxygen vacancies as follows:

Snq4 2eyOy2 V sV q2ey 1Ž .Ž .b 2yb o d

The oxygen vacancies of SnO , however, reacted with2

an ambient oxygen gas at operation temperature, and thevacancy concentration varied toward an equilibrium statuscompatible with the ambient oxygen pressure and tempera-ture. Since equilibrium oxygen partial pressure with SnO2

is extremely low from a thermodynamic point of view,ambient oxygen partial pressure of about 0.2 atm causedthe forward reaction of the following equation, and thus,diminished the oxygen vacancies in SnO :2

V q1r2O g sO 2Ž . Ž .o 2 o

For this reason, the base resistance of WO -mixed SnO3 2

kept increasing with the sensor operating condition. Inaddition, the resistance of SnO steadily increased with2

operating time. On the other hand, the conduction mecha-nism of W-doped SnO gas sensor was dominated by2

Ž . Ž .Fig. 7. Comparative sensitivities DV of two types of sensors to several gases DVsV yV .out,gas out,air


Ž . Ž . Ž . Ž . Ž .Fig. 8. Voltage responses of W 0.1 mol% -doped SnO and WO 5 wt.% -mixed SnO sensors to a 300 ppm acetone, b 500 ppm alcohol, c2 3 2

cigarette smoke under air-ambient.

controlled valence mechanism instead of nonstoichiometryas follows:

Snq4 2eyWq6 Oy2 3Ž .1yx x x 2

This means that the resistance of W-doped SnO gas2

sensor was controlled mainly by the dose of tungstenwithout large regarding of oxygen vacancies concentration.Even though the oxygen vacancies remained in SnO2


reacted with ambient oxygen gas, the effect was not enoughas to change the whole resistance of W-doped SnO2

sensor. Consequently, the long-term stability could beobtained.

The fluctuations of V voltage in Fig. 6 were consid-out

ered to be attributed to the variation of temperature andhumidity of ambient air during sensor operation. Theambient air was changed from 58C to 208C in temperature

Ž .and from 30% to 70% in relative humidity R.H .

3.2. SensitiÕity

Fig. 7 shows the relative sensitivities of two types ofsensors to pollutant gases such as acetone, alcohol, and

Ž .cigarette smoke. In this figure, W 0.1 mol% -doped SnO2

gas sensor had higher sensitivities to both acetone andŽ .alcohol compared with WO 5 wt.% -mixed SnO gas3 2

sensor. This result was considered to be due to the smallerŽ .grain size of our coprecipitated powder ;20 nm than

one of the commercial SnO powder for the WO -mixed2 3Ž . ŽSnO gas sensor ;40 nm . On the other hand, WO 52 3

.wt.% -mixed SnO gas sensor showed higher sensitivity to2

cigarette smoke. WO material has specially high sensitiv-3

ity to H S gas, which is one of principal elements of2

cigarette smoke. The following equation shows the reac-w xtion of WO with H S gas 13 .3 2

3WO s q7H S g ™3WS s qSO g q7H O gŽ . Ž . Ž . Ž . Ž .3 2 2 2 2

4Ž .

For our sensors the oxygen vacancies were created bythe reaction of WO with H S, and thus, the resistance of3 2

Žthe gas sensors largely decreased. For this reason, WO 53.wt.% -mixed SnO gas sensor showed higher sensitivity2

due to the comparatively larger W content compared withŽ .W 0.1 mol% -doped SnO gas sensor.2

Ž .Fig. 8 shows the voltage responses of W 0.1 mol% -Ž .doped SnO and WO 5 wt.% -mixed SnO sensors to2 3 2

300 ppm acetone, 500 ppm alcohol, and cigarette smoke.Both sensors immediately responded to all the gases. How-ever, W-doped SnO sensor showed shorter recovery time2

Ž .than the WO 5 wt.% -mixed SnO sensor. It was found3 2

that cigarette smoke retarded the recovery of the sensor. Itseems that this was because cigarette smoke was not likelyto desorb from the sensor surface.

4. Conclusion

Through this study, we believe that the small amount ofŽ .W ;0.1 mol% -doping into SnO improved long-term2

stability, which has been a principal problem concerning

SnO semiconductor gas sensors. The W-doped SnO2 2

sensing material could be successfully prepared by copre-Ž .cipitation. W 0.1 mol% -doped SnO gas sensor fabri-2

cated in this work showed higher gas sensitivity to bothŽ .acetone and alcohol compared with WO 5 wt.% -mixed3

Ž .SnO gas sensor. The WO 5 wt.% -mixed SnO gas2 3 2

sensor, however, showed superior sensitivity to cigarettesmoke due to the larger W content. It is expected that gassensors possessing the excellent sensing performance topollutant gases can be obtained by the adjustment of totalWO content with a small amount of W doping into SnO .3 2

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Biographies

Jong Hyun Park received his BS in Inorganic Materials Engineering fromPusan National University in 1997. Now, he is a graduate student of theDepartment of Inorganic Materials Engineering in Pusan National Univer-sity.


Kwang Ho Kim received his BS in Metallurgical Engineering from SeoulNational University in 1980, and his MS and PhD degrees in MaterialsScience and Engineering from Korea Advanced Institute of Science and

Ž .Technology KAIST in 1982 and 1986, respectively. Since 1985, he hasbeen with the Department of Inorganic Materials Engineering in PusanNational University, first as a full-time lecturer and presently as aprofessor. His main research areas are thin-film processing and electronicceramics.

improvement of long-term stability in sno2-based gas sensor for monitoring offensive odor

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