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Sensitivity to oxygen and response characteristics of nanocrystalline SnO2 at room temperature
Nrmdmdud~.
Pergamon
Vol. 10.No. 1.pp. 5563.1998 Elsevie? sciala Lid 0 1998ActaMebllurgia Inc. RinteclmtheUsA. AllrighI8rcscxved 0969773198$19.00+ .oo
PII SO965-9773(98)00024-S
SENSITIVITY TO OXYGEN AND RESPONSE CHARACTERISTICS OF NANOCRYSTALLINE SnO2 AT ROOM TEMPERATURE J.X. Zhang*, Y.X. IA*, CM. Shek**, and J.K.L. Lai”
*Department of Physics, Zhongshan University, Guangzhou, 510275, China **Department of Physics and Materials Science, City University of Hong Kong, Hong Kong (AcceptedDecember 21,1997) Abstract - The electricalproperties of compressed nanoctystallinetin (IV) oxide were studied.It wasobserved thatthe room temperatureelectricalresistanceof nanocrystallineSnO2 samples increased withincreasing partialpressure of oxygen. In addition,when there is a sudden change in thleoxygen partial pressure, a shortpulse of spontaneouselectromotiveforce of the millivoltorder can be detected. Thisnewfindinghas the potential applicationof leakage detection in vacuum systemsor pressure vessels. 01998 Acta MetallurgicaInc. INTRODUCTION
It is well known that there are four different adsorption states of oxygen on the surface of Sn02 crystals, namely, @,@‘. 00 and &-. In the last three types of adsorption state, electrons have to be transferred from Sn@ to the oxygen atoms or molecules to form the ions. The electrons are supplied from the conduction band of the SnO2 crystal, and it follows that the conductance of a SnO2 crystal will change as the adsorption state of oxygen changes (l-4). This peculiar property of Sn@ has been utilized in microelectronics, optoelectronics and gas sensors (45). Since ai pure Sn@ crystal is a wide forbidden band-gap n-type semi-conductor (Es = 3.6 eV) (5), the room temperature electron density in the conduction band of large crystals is small. This gas-sensing property is, therefore, only prominent at high temperatures, e.g. above 150°C. In fact, all currently iavailable SnOz gas sensors require pm-heating to the operating temperature, usually above 200°C. This limits the applications and causes inconvenience for the usage of this material as a gas sensor. (6-8) The proportion of grain boundaries on the crystal surface is larger in nanocrystalline SnO2, compared with conventional materials, and it is known that the band-gap energy and the condition of tbe dangling bonds in the grain boundary deviate from those in the crystal surface (6). These may possibly affect the interaction between oxygen and the Sn@ surface, so that oxygen adsorption can occur at lower temperature, say room temperature, and induce changes in the resistivity of the SnOz. Furthermore, the adhesion between the adsorption states, @-, O- or aand the dangling bonds on nanocrystalline SnO2 is weaker than that with the dangling bonds in the crystal lattice (6). This will affect the response of the nanocrystalline Sn@ to the partial pressure of gases at room temperature. 55
56
JX ZHANG,YX LI, CH SHEK ANDJKL IA
This paper reports the results of the investigation on the oxygen-sensitive characteristics of nanocrystalline Sri@ at room temperature. These include the variation of room temperature resistivity of a nanocrystalline Snadevice with oxygen partial pressure and the transient response of the resistivity or spontaneous electro-motive force to sudden changes of oxygen partial pressure. It was found that the room temperature resistivity of the nanocrystalline Sn02 increases with increasing oxygen partial pressure. Moreover, a short pulse of electromotive force in the millivolt range can be detected if the partial pressure of oxygen changes abruptly. This feature may find applications in leakage detection in vacuum systems or pressure vessels. EXPERIMENTAL The nanocrystalline SnGz sample was prepared through hydrolysis of tin chloride (SnC12.2H20). Granular solid SnC12.2H20 was fmt dissolved in concentrated hydrochloric acid and then ammonia solution was dropped slowly into the acidic solution until the pH value of the whole solution became 6.0-6.5. The beaker containing the solution was immersed at first in a container with ice-water for 1 hour and then in an ultrasonic cleaner at 0°C. The solution was kept at 0°C and agitated vigorously. Sn(OH)2 particles, the size of which depends on the temperature of the solution, were precipitated. The lower the temperature, the smaller the particle size was, and nanocrystalline Sn(OI-I)2 can be obtained at room temperature. The precipitated Sn(OI-I)2 was washed with water, then with ethanol, and fmally with ether until no chloride ions could be detected. The dried tin hydroxide powder was heated gently with an infrared lamp to thermally decompose the hydroxide at a low temperature (400°C). The resulting product was a black powder of nanocrystalline SnO which was subsequently oxidized to Sn@ by blowing oxygen over the SnO powder. Pure Sna has a white color but the nanocrystalline SnO.Lproduced by these procedure was grayish white. Therefore, it was thought to be nonstoichiometric and had a formula Sn@a It can be seen from Figure 1, which is a transmission
Figure 1. TEM photograph of the nano-Snob particles.
:SENSITMW
1000
TO
OXYGEN AND RESPONSE CHARACTERISTICS OF NANOCFWSTAUINE
SnO,
57
--n-
900 800 700 600 z V
500 400 300 200 100 60
70
80
28
Figure 2. X-ray diffraction spectrum of nanocrystalline Sri@.
electron micrograph of the SnOz powder, that the powder had a narrow distribution of sixes (8 15 nm) and was well dispersed. Figure 2 is the result of x-ray diffraction of the nanocrystalline SnOz with a Rigaku D/max x-ray diffractometer. The major peaks were all indexed and the Sn@ powder was found to have the rutile structure. Thin disks of Sn&, with diameter 10 mm and approximately 1 mm thick, were made by compressing the SnOz powder in a hydraulic press at 200 MPa for 5 minutes. Electrical contacts were made on the disks using silver wires and silver paste. The electrical resistivity of the sensor element was rneasured with the four-probe method, and it was found that when the resistance of the disk is about several hundred ohms, the sensitivity to oxygen is better. In order to investigate the effects of the partial pressures of oxygen and other gases as well as the temperature, the disk samples were put into a vacuum heating chamber while doing electrical measurements. EXPERIMENTAL
RESULTS
Figure 3 shows the voltage-current characteristics of the nanocrystalline SnO;! gas sensor element measured in air at room temperature and at 15O“C. Measurements were carried out with the voltage applied across contacts A and D, and they varied from 0 to 200 mV. Both lines in the figure are linear and are passing through origin. Actually, it was found in the experiment that the V-I relationship of the sensor element remains linear from room temperature up to 250°C while V 5 1V. This indicates that good ohmic junctions were formed between the silver wires and the SnOz disks. By gradually increasing the temperature in the vacuum heating chamber while keeping the chamber open to the ambient, tbe variation of resistivity of tbe sensor element with temperature in air was studied and the results were shown in Figure 4. It can be seen from tbe figure that the resistance decreased slightly from room temperature to roughly 6O”C,and then the trend changed
JX BANG, YX LI, CH SHEK AND JKL
58
LAI
1
0
20
40
60
80
100 120 140 160 180 200 V,mV
Figure 3. V-I characteristicsof nanoSn02. gas sensor in air at room temperature.
400
!
1’
a
"
1
200 -
LOO -
0 0
200
100
I 300
T," C
Figure 4. R-T curve of nano-SnOzgas sensor in air.
to an increasing one up to 170°C,at which the resistance was the largest in the temperature range studied. When the temperature was further increased, the resistance of the sensor decreased to a minimum at about 275”C, and two small sharp peaks can be seen in the temperature range 275 300°C. Thevariationsoftheresistanceofthesensorfortemperaturesbelow275”Cweretheresults of the temperature dependence of the equilibriumamong the four oxygen adsorption states on the SnO.l surface. (9,lO)
&a-iw~
TO &atx~
100
50
AND~PoWE ~~ARAOTERI~TIOS OF NANOWIYSTA~JNE &IO,
59
-
0
100
200
300
400
500
600
700
800
Partial Pressure of Oxygen (Torr)
Figure 5. R vs. Pe (oxygen partial pressure) plot at mom temperature. The heating chamber was then evacuated at room temperature. Oxygen was gradually filled into the chamber to attain the desired oxygen partial pressure. At every set value of oxygen partial pressure Po2, the equilibrium value of resistance of the sensor element was measured after approximatel:ythree minutes. Figure 5 shows the variations of the sensor resistance, R, with oxygen partial pressure in the range 0.5 torr to 760 torr (i.e. atmospheric pressure). The resistance increased slowly with PQ when P% was small. After PO* was increased beyond approximately 200 torr, R and PO, follow a more or less linear relationship.The resistance increased monotonically with PoZ from 110at 0.5 torr to 264 at 760 torr, i.e. R increased 2.4 times in the studied range of Pq The reqonse of the sensor to the concentration of reducing gases (combustible gases) was also investigated. Figure 6 shows the changes of sensor resistance with the concentrations of carbon monoxide (CO) and ethanol (CzH5OH)in air at 270°C. The resistance decreases with increasing concentration of both CO and CzHsOH,but the sensitivity to CO is higher than that to C2HsOH ( 11:). The me~urements described above were done in the static condition, viz., the resistance values were rneasured after it had settled to a steady value with time. However, some interesting phenomena were observed in the transient response of the St@ sensor when there were sudden changes of oxygen partial pressure. Figure 7 is the record of the transient response of a sensor element to sudden changes of PO2 in the vacuum heating chamber. When the chamber was evacuated, a pulse of spontaneous electromotive force (EMF)of the order of millivolt was detected across the sensor element, Figure 7(a). The response time, taken as the time required to rise to a maximum output, of the spontaneous EMF is less than 10 seconds. After the EMF had reached a steady value, sudden airing into the chamber also induced a spontaneousEMF,which was slightly weaker than that shown in Figure 7(a), and the direction of which was reversed, Figure 7(b).
60
JXZHANG,YXLI,CHSHEK~DJKLLAI
0,"' 0
I 400
200
600 Gas
I 800
I 1000
I.,,,,,, 1200 1400 1600
Concertration,
1800 2000
ppm
Figure 6. Plot of resistance R vs. concentration of reducing gas CO and C2H50H at 270°C.
0.;
-,
3
0.6 c 0.5 -
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> E S
0.4
-
0.3
-
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-
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OS >
E
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I
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-0.1 -
-0.3 -0.4
O
10
C 20
‘.’ 30
40
L 50
I 60
70
#‘I80
90
t. set
Figure 7. Spontaneous EhJF vs. time curves at room temperature: (a) evacuating, and (b) airing.
10
20
30
t, set
Figure 8. Response curve (spontaneous EMF vs. time) of nano-Sn@ gas sensor while blowing a mixture of CO2 and H20 (steam).
!~ENSITIWM TO OXYQEN AND RESPONSE
Cmmcrmsncs OFNANOCFIYSTAUINE !&IO,
61
Besides, the response time of the EMF to airing is only of the order of seconds. For comparison, Sn02 samples with grain sizes of the order of micrometer were also tested. The transient responses during both evacuating and airing were in the same direction and the response time was approximately 45 seconds. The spontaneous EMF was only of the order of microvolt, which is comparable in magnitude to thermal noises. In order to distinguish whether the spontaneous EMF was due to the abrupt change in oxygen concentration or the mechanical action of the air flow, a further test was done by blowing a mixture of C@ and H20 (steam) onto the sensor element at a rate of roughly 1 liter per minute. The resulting response is shown in Figure 8. It is obvious from the figure that the response to the blowing test is very similar to that shown in Figure 7 except that the direction of the EMF pulse was reversed. This indicates that the spontaneous EMF may be attributed to the change in the oxygen adsorption states on the surface of the sensor element due to the air flow. However, if a reducing gas (e.g. liquefied petroleum gas) was blown over the specimen, only a lesser spontaneous EMF can be observed. DISCUSSION Compressed Sn@ powder is of great interest in semiconductor sensors. The resistance of a compressed Sn02 powder pellet depends strongly on gas adsorption (9). Perfect crystal of tetragonal Snth is a wide band gap semiconductor. Nevertheless, with the presence of oxygen vacancies due to oxygen-deficiency, the semiconductor becomes n-type with a donor level of only 0.03-0.15 eV. The binding energy of oxygen adsorbed on the SnO.Lsurface (or surface energy level) is 0.3-0.6 eV (12), depending on the adsorption state of the oxygen. When an oxygen molecule or atom is adsorbed onto the surface of the Sn% pellet and becomes an ion of O-, @or a-. the ellectrontransfers from conduction band to adsorbed oxygen ion, and therefore the iesistance of the compressed Sn02 powder pellet increases. In view of this, when different gas species (oxidizing, reducing or idle) are adsorbedon a Sn@ surface, the energy level on the surface of the crystal will change and this induces changes in the resistivity of the Sn02 crystal. Nanocrystalline Sn0.z has a much larger fraction of surface atoms. This can account for the larger number of tin atom positions being deviated from the perfect lattice and also the higher proportion of oxygen vacalncies compared with materials with normal grain sizes. This feature of nanocrystalline Sn02 gives rir;e to a higher gas sensitivity than that of conventional materials. Fromthf:experimentalresultsofthisinvestigation,thegassensingpropertiesofnanocrystalline Sn02 at room temperature is more or less limited to oxygen. Figure 5 and 6 illustrated that the equilibrium resistance of the Sn@ sensing element was not affected by the concentration of CO and QHsOH, which are both reducing gases. The spontaneous EMF as shown in Figure 7 and 8 is thought to be due to sudden changes of oxygen partial pressure. Theconcentrations of chemically inertgas (e.g. Co2) or reducing gases (e.g. LPG) do not have sufficient effect on the SnOz, and only a less obvious spontaneous EMF will be induced by sudden changes in their partial pressures. Nonetheless, if steam is added into C@, then a pulse of spontaneous EMF can be observed, as shown in Figure 8. This supports the idea that the nanocrystalline Sn@ is sensitive to the oxygen or OH- in steam, which could be an oxidizing gas. The spontaneous EMF observed in this investigation is thought to be due to the adjustment of the states of the surface adsorbed oxygen in response to sudden changes in the ambient oxygen
62
JX ZHANG,YX LI, CH SHEK ANDJKL IA
0.10lr
stop blowing 0.05
2
0.00
% %
-0.05
-0.10
-0.1 s
I
T I
10
20
30
I.
8,
I
40
50
60
._
IO
t, set
Figure 9. Response curve of nanoSt@ gas sensor while blowing CO2 gas.
concentration. When the ambient PO2 changes abruptly, the equilibrium between the oxygen in the environment and the surface adsorbed oxygen is upset. The amount and distribution of adsorbed oxygen among the different states Q&,0-, @- and 02) have to be adjusted to reach a new equilibrium. This induces changes in surface resistivity of the Sn& as well as exchange of charge between SnOz and the adsorbed oxygen. Spontaneous EMF is thus resulted. If (202, a chemically inert gas, is blown over the surface of Sn&, the flow of gas can only mechanically disturb the concentration of oxygen on the surface and produce a small and broad pulse of spontaneous EMF. Once this has stopped blowing, the sensor element can quickly restore to its original state (spontaneous Eh4F = 0), Figure 9. This is because the distribution of the adsorbed oxygen is less affected by the flow of an chemically inert gas.
CONCLUSIONS It was found from the experiment that compressed nanocrystalline SnO2 powder demonstrates an appreciable sensitivity to oxygen at room temperature. Furthermore, when the partial pressure of oxygen or other oxidizing gases in the environment changes suddenly, there is a spontaneous electromotive force of the order of millivolt induced in the nanocrystalline Sri@. This feature may be applied in leak detection in pressure vessels or in monitoring of vacuum systems.
1. 2. 3.
Chemical Sensor Technology, ed. N. Yamawe, vol. 3, Kodansha Ltd., Tokyo, 1991. Koh. D., Sensors andActuators, 1989,18,71. Cox, F., Fryberger, T.B. and Semancik, S., Physical Review, 1988, B38,2072.
SENSITIVITYTO OXYGEN AND RESPONSECHARACTERISTICS OFNANOCRYSTALUNESnO,
4. 5. 6. 7. 8. 9. 10. 11. 12.
63
Mulheran, P.A., Harding, J.H., Modeling Simulations in Materials Science andEngineering, 1992, 1.39. Nie, W.J., Advanced Materials, 1993.5.520. Gtipel, W. and Schierbaum, K.D., Sensors and Actuators, 1995, B26-27,1. For example, see Figaro Inc., Japan, Tech. Rep. Jhokura, K. and Watson, J., The Stannic Oxia’e Gas Sensors4’rinciples and Applications, CRC Press, B’oca Raton, FL, 1994. Qu, B.D., Acta Inorganic Materials, 1991,6,321. Madou, M.J. and Morrison, S.R., Chemical Sensing with Solid State Devices, Academic Press, 1989, chl. 2 and 3. Davydov, A.A., Journal ofApplied Spectroscopy, 19!92,56,365. Yi, H.Z., Functional Materials, 1991,22,286.
Pergamon
Vol. 10.No. 1.pp. 5563.1998 Elsevie? sciala Lid 0 1998ActaMebllurgia Inc. RinteclmtheUsA. AllrighI8rcscxved 0969773198$19.00+ .oo
PII SO965-9773(98)00024-S
SENSITIVITY TO OXYGEN AND RESPONSE CHARACTERISTICS OF NANOCRYSTALLINE SnO2 AT ROOM TEMPERATURE J.X. Zhang*, Y.X. IA*, CM. Shek**, and J.K.L. Lai”
*Department of Physics, Zhongshan University, Guangzhou, 510275, China **Department of Physics and Materials Science, City University of Hong Kong, Hong Kong (AcceptedDecember 21,1997) Abstract - The electricalproperties of compressed nanoctystallinetin (IV) oxide were studied.It wasobserved thatthe room temperatureelectricalresistanceof nanocrystallineSnO2 samples increased withincreasing partialpressure of oxygen. In addition,when there is a sudden change in thleoxygen partial pressure, a shortpulse of spontaneouselectromotiveforce of the millivoltorder can be detected. Thisnewfindinghas the potential applicationof leakage detection in vacuum systemsor pressure vessels. 01998 Acta MetallurgicaInc. INTRODUCTION
It is well known that there are four different adsorption states of oxygen on the surface of Sn02 crystals, namely, @,@‘. 00 and &-. In the last three types of adsorption state, electrons have to be transferred from Sn@ to the oxygen atoms or molecules to form the ions. The electrons are supplied from the conduction band of the SnO2 crystal, and it follows that the conductance of a SnO2 crystal will change as the adsorption state of oxygen changes (l-4). This peculiar property of Sn@ has been utilized in microelectronics, optoelectronics and gas sensors (45). Since ai pure Sn@ crystal is a wide forbidden band-gap n-type semi-conductor (Es = 3.6 eV) (5), the room temperature electron density in the conduction band of large crystals is small. This gas-sensing property is, therefore, only prominent at high temperatures, e.g. above 150°C. In fact, all currently iavailable SnOz gas sensors require pm-heating to the operating temperature, usually above 200°C. This limits the applications and causes inconvenience for the usage of this material as a gas sensor. (6-8) The proportion of grain boundaries on the crystal surface is larger in nanocrystalline SnO2, compared with conventional materials, and it is known that the band-gap energy and the condition of tbe dangling bonds in the grain boundary deviate from those in the crystal surface (6). These may possibly affect the interaction between oxygen and the Sn@ surface, so that oxygen adsorption can occur at lower temperature, say room temperature, and induce changes in the resistivity of the SnOz. Furthermore, the adhesion between the adsorption states, @-, O- or aand the dangling bonds on nanocrystalline SnO2 is weaker than that with the dangling bonds in the crystal lattice (6). This will affect the response of the nanocrystalline Sn@ to the partial pressure of gases at room temperature. 55
56
JX ZHANG,YX LI, CH SHEK ANDJKL IA
This paper reports the results of the investigation on the oxygen-sensitive characteristics of nanocrystalline Sri@ at room temperature. These include the variation of room temperature resistivity of a nanocrystalline Snadevice with oxygen partial pressure and the transient response of the resistivity or spontaneous electro-motive force to sudden changes of oxygen partial pressure. It was found that the room temperature resistivity of the nanocrystalline Sn02 increases with increasing oxygen partial pressure. Moreover, a short pulse of electromotive force in the millivolt range can be detected if the partial pressure of oxygen changes abruptly. This feature may find applications in leakage detection in vacuum systems or pressure vessels. EXPERIMENTAL The nanocrystalline SnGz sample was prepared through hydrolysis of tin chloride (SnC12.2H20). Granular solid SnC12.2H20 was fmt dissolved in concentrated hydrochloric acid and then ammonia solution was dropped slowly into the acidic solution until the pH value of the whole solution became 6.0-6.5. The beaker containing the solution was immersed at first in a container with ice-water for 1 hour and then in an ultrasonic cleaner at 0°C. The solution was kept at 0°C and agitated vigorously. Sn(OH)2 particles, the size of which depends on the temperature of the solution, were precipitated. The lower the temperature, the smaller the particle size was, and nanocrystalline Sn(OI-I)2 can be obtained at room temperature. The precipitated Sn(OI-I)2 was washed with water, then with ethanol, and fmally with ether until no chloride ions could be detected. The dried tin hydroxide powder was heated gently with an infrared lamp to thermally decompose the hydroxide at a low temperature (400°C). The resulting product was a black powder of nanocrystalline SnO which was subsequently oxidized to Sn@ by blowing oxygen over the SnO powder. Pure Sna has a white color but the nanocrystalline SnO.Lproduced by these procedure was grayish white. Therefore, it was thought to be nonstoichiometric and had a formula Sn@a It can be seen from Figure 1, which is a transmission
Figure 1. TEM photograph of the nano-Snob particles.
:SENSITMW
1000
TO
OXYGEN AND RESPONSE CHARACTERISTICS OF NANOCFWSTAUINE
SnO,
57
--n-
900 800 700 600 z V
500 400 300 200 100 60
70
80
28
Figure 2. X-ray diffraction spectrum of nanocrystalline Sri@.
electron micrograph of the SnOz powder, that the powder had a narrow distribution of sixes (8 15 nm) and was well dispersed. Figure 2 is the result of x-ray diffraction of the nanocrystalline SnOz with a Rigaku D/max x-ray diffractometer. The major peaks were all indexed and the Sn@ powder was found to have the rutile structure. Thin disks of Sn&, with diameter 10 mm and approximately 1 mm thick, were made by compressing the SnOz powder in a hydraulic press at 200 MPa for 5 minutes. Electrical contacts were made on the disks using silver wires and silver paste. The electrical resistivity of the sensor element was rneasured with the four-probe method, and it was found that when the resistance of the disk is about several hundred ohms, the sensitivity to oxygen is better. In order to investigate the effects of the partial pressures of oxygen and other gases as well as the temperature, the disk samples were put into a vacuum heating chamber while doing electrical measurements. EXPERIMENTAL
RESULTS
Figure 3 shows the voltage-current characteristics of the nanocrystalline SnO;! gas sensor element measured in air at room temperature and at 15O“C. Measurements were carried out with the voltage applied across contacts A and D, and they varied from 0 to 200 mV. Both lines in the figure are linear and are passing through origin. Actually, it was found in the experiment that the V-I relationship of the sensor element remains linear from room temperature up to 250°C while V 5 1V. This indicates that good ohmic junctions were formed between the silver wires and the SnOz disks. By gradually increasing the temperature in the vacuum heating chamber while keeping the chamber open to the ambient, tbe variation of resistivity of tbe sensor element with temperature in air was studied and the results were shown in Figure 4. It can be seen from tbe figure that the resistance decreased slightly from room temperature to roughly 6O”C,and then the trend changed
JX BANG, YX LI, CH SHEK AND JKL
58
LAI
1
0
20
40
60
80
100 120 140 160 180 200 V,mV
Figure 3. V-I characteristicsof nanoSn02. gas sensor in air at room temperature.
400
!
1’
a
"
1
200 -
LOO -
0 0
200
100
I 300
T," C
Figure 4. R-T curve of nano-SnOzgas sensor in air.
to an increasing one up to 170°C,at which the resistance was the largest in the temperature range studied. When the temperature was further increased, the resistance of the sensor decreased to a minimum at about 275”C, and two small sharp peaks can be seen in the temperature range 275 300°C. Thevariationsoftheresistanceofthesensorfortemperaturesbelow275”Cweretheresults of the temperature dependence of the equilibriumamong the four oxygen adsorption states on the SnO.l surface. (9,lO)
&a-iw~
TO &atx~
100
50
AND~PoWE ~~ARAOTERI~TIOS OF NANOWIYSTA~JNE &IO,
59
-
0
100
200
300
400
500
600
700
800
Partial Pressure of Oxygen (Torr)
Figure 5. R vs. Pe (oxygen partial pressure) plot at mom temperature. The heating chamber was then evacuated at room temperature. Oxygen was gradually filled into the chamber to attain the desired oxygen partial pressure. At every set value of oxygen partial pressure Po2, the equilibrium value of resistance of the sensor element was measured after approximatel:ythree minutes. Figure 5 shows the variations of the sensor resistance, R, with oxygen partial pressure in the range 0.5 torr to 760 torr (i.e. atmospheric pressure). The resistance increased slowly with PQ when P% was small. After PO* was increased beyond approximately 200 torr, R and PO, follow a more or less linear relationship.The resistance increased monotonically with PoZ from 110at 0.5 torr to 264 at 760 torr, i.e. R increased 2.4 times in the studied range of Pq The reqonse of the sensor to the concentration of reducing gases (combustible gases) was also investigated. Figure 6 shows the changes of sensor resistance with the concentrations of carbon monoxide (CO) and ethanol (CzH5OH)in air at 270°C. The resistance decreases with increasing concentration of both CO and CzHsOH,but the sensitivity to CO is higher than that to C2HsOH ( 11:). The me~urements described above were done in the static condition, viz., the resistance values were rneasured after it had settled to a steady value with time. However, some interesting phenomena were observed in the transient response of the St@ sensor when there were sudden changes of oxygen partial pressure. Figure 7 is the record of the transient response of a sensor element to sudden changes of PO2 in the vacuum heating chamber. When the chamber was evacuated, a pulse of spontaneous electromotive force (EMF)of the order of millivolt was detected across the sensor element, Figure 7(a). The response time, taken as the time required to rise to a maximum output, of the spontaneous EMF is less than 10 seconds. After the EMF had reached a steady value, sudden airing into the chamber also induced a spontaneousEMF,which was slightly weaker than that shown in Figure 7(a), and the direction of which was reversed, Figure 7(b).
60
JXZHANG,YXLI,CHSHEK~DJKLLAI
0,"' 0
I 400
200
600 Gas
I 800
I 1000
I.,,,,,, 1200 1400 1600
Concertration,
1800 2000
ppm
Figure 6. Plot of resistance R vs. concentration of reducing gas CO and C2H50H at 270°C.
0.;
-,
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> E S
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E
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b ‘f-;
-0.1 -
-0.3 -0.4
O
10
C 20
‘.’ 30
40
L 50
I 60
70
#‘I80
90
t. set
Figure 7. Spontaneous EhJF vs. time curves at room temperature: (a) evacuating, and (b) airing.
10
20
30
t, set
Figure 8. Response curve (spontaneous EMF vs. time) of nano-Sn@ gas sensor while blowing a mixture of CO2 and H20 (steam).
!~ENSITIWM TO OXYQEN AND RESPONSE
Cmmcrmsncs OFNANOCFIYSTAUINE !&IO,
61
Besides, the response time of the EMF to airing is only of the order of seconds. For comparison, Sn02 samples with grain sizes of the order of micrometer were also tested. The transient responses during both evacuating and airing were in the same direction and the response time was approximately 45 seconds. The spontaneous EMF was only of the order of microvolt, which is comparable in magnitude to thermal noises. In order to distinguish whether the spontaneous EMF was due to the abrupt change in oxygen concentration or the mechanical action of the air flow, a further test was done by blowing a mixture of C@ and H20 (steam) onto the sensor element at a rate of roughly 1 liter per minute. The resulting response is shown in Figure 8. It is obvious from the figure that the response to the blowing test is very similar to that shown in Figure 7 except that the direction of the EMF pulse was reversed. This indicates that the spontaneous EMF may be attributed to the change in the oxygen adsorption states on the surface of the sensor element due to the air flow. However, if a reducing gas (e.g. liquefied petroleum gas) was blown over the specimen, only a lesser spontaneous EMF can be observed. DISCUSSION Compressed Sn@ powder is of great interest in semiconductor sensors. The resistance of a compressed Sn02 powder pellet depends strongly on gas adsorption (9). Perfect crystal of tetragonal Snth is a wide band gap semiconductor. Nevertheless, with the presence of oxygen vacancies due to oxygen-deficiency, the semiconductor becomes n-type with a donor level of only 0.03-0.15 eV. The binding energy of oxygen adsorbed on the SnO.Lsurface (or surface energy level) is 0.3-0.6 eV (12), depending on the adsorption state of the oxygen. When an oxygen molecule or atom is adsorbed onto the surface of the Sn% pellet and becomes an ion of O-, @or a-. the ellectrontransfers from conduction band to adsorbed oxygen ion, and therefore the iesistance of the compressed Sn02 powder pellet increases. In view of this, when different gas species (oxidizing, reducing or idle) are adsorbedon a Sn@ surface, the energy level on the surface of the crystal will change and this induces changes in the resistivity of the Sn02 crystal. Nanocrystalline Sn0.z has a much larger fraction of surface atoms. This can account for the larger number of tin atom positions being deviated from the perfect lattice and also the higher proportion of oxygen vacalncies compared with materials with normal grain sizes. This feature of nanocrystalline Sn02 gives rir;e to a higher gas sensitivity than that of conventional materials. Fromthf:experimentalresultsofthisinvestigation,thegassensingpropertiesofnanocrystalline Sn02 at room temperature is more or less limited to oxygen. Figure 5 and 6 illustrated that the equilibrium resistance of the Sn@ sensing element was not affected by the concentration of CO and QHsOH, which are both reducing gases. The spontaneous EMF as shown in Figure 7 and 8 is thought to be due to sudden changes of oxygen partial pressure. Theconcentrations of chemically inertgas (e.g. Co2) or reducing gases (e.g. LPG) do not have sufficient effect on the SnOz, and only a less obvious spontaneous EMF will be induced by sudden changes in their partial pressures. Nonetheless, if steam is added into C@, then a pulse of spontaneous EMF can be observed, as shown in Figure 8. This supports the idea that the nanocrystalline Sn@ is sensitive to the oxygen or OH- in steam, which could be an oxidizing gas. The spontaneous EMF observed in this investigation is thought to be due to the adjustment of the states of the surface adsorbed oxygen in response to sudden changes in the ambient oxygen
62
JX ZHANG,YX LI, CH SHEK ANDJKL IA
0.10lr
stop blowing 0.05
2
0.00
% %
-0.05
-0.10
-0.1 s
I
T I
10
20
30
I.
8,
I
40
50
60
._
IO
t, set
Figure 9. Response curve of nanoSt@ gas sensor while blowing CO2 gas.
concentration. When the ambient PO2 changes abruptly, the equilibrium between the oxygen in the environment and the surface adsorbed oxygen is upset. The amount and distribution of adsorbed oxygen among the different states Q&,0-, @- and 02) have to be adjusted to reach a new equilibrium. This induces changes in surface resistivity of the Sn& as well as exchange of charge between SnOz and the adsorbed oxygen. Spontaneous EMF is thus resulted. If (202, a chemically inert gas, is blown over the surface of Sn&, the flow of gas can only mechanically disturb the concentration of oxygen on the surface and produce a small and broad pulse of spontaneous EMF. Once this has stopped blowing, the sensor element can quickly restore to its original state (spontaneous Eh4F = 0), Figure 9. This is because the distribution of the adsorbed oxygen is less affected by the flow of an chemically inert gas.
CONCLUSIONS It was found from the experiment that compressed nanocrystalline SnO2 powder demonstrates an appreciable sensitivity to oxygen at room temperature. Furthermore, when the partial pressure of oxygen or other oxidizing gases in the environment changes suddenly, there is a spontaneous electromotive force of the order of millivolt induced in the nanocrystalline Sri@. This feature may be applied in leak detection in pressure vessels or in monitoring of vacuum systems.
1. 2. 3.
Chemical Sensor Technology, ed. N. Yamawe, vol. 3, Kodansha Ltd., Tokyo, 1991. Koh. D., Sensors andActuators, 1989,18,71. Cox, F., Fryberger, T.B. and Semancik, S., Physical Review, 1988, B38,2072.
SENSITIVITYTO OXYGEN AND RESPONSECHARACTERISTICS OFNANOCRYSTALUNESnO,
4. 5. 6. 7. 8. 9. 10. 11. 12.
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Mulheran, P.A., Harding, J.H., Modeling Simulations in Materials Science andEngineering, 1992, 1.39. Nie, W.J., Advanced Materials, 1993.5.520. Gtipel, W. and Schierbaum, K.D., Sensors and Actuators, 1995, B26-27,1. For example, see Figaro Inc., Japan, Tech. Rep. Jhokura, K. and Watson, J., The Stannic Oxia’e Gas Sensors4’rinciples and Applications, CRC Press, B’oca Raton, FL, 1994. Qu, B.D., Acta Inorganic Materials, 1991,6,321. Madou, M.J. and Morrison, S.R., Chemical Sensing with Solid State Devices, Academic Press, 1989, chl. 2 and 3. Davydov, A.A., Journal ofApplied Spectroscopy, 19!92,56,365. Yi, H.Z., Functional Materials, 1991,22,286.