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The interaction of different oxidizing agents on doped tin oxide
Sensors and Actuators B 24-25 (1995) 512415
The interaction of different oxidizing agents on doped tin oxide I. Sayago a~*,J.
Gutihrez a, L. Ads a, J.I. Robla a, M.C. Horrillo ‘, J. Getino a, J.A. Agapito b
’ Laboratoriode Sensors, CSIC, Semmo, 144, 28006~Madtid,Spain bDepartamentode Eiechdnica, Facultad CienciasFGxs, UnivwsidadComplutense,Ciudad Universitaria, 28040.Madrid Spain
Abstract The interaction of NO*, NO= and O2 on an SnO, thin-film surface is studied. Experiments have been carried out for different tin-oxide films, undoped and doped with Pt, In and Al, prepared by reactive sputtering (r.f.) on alumina substrates. These films should allow the development of detectors for nitrogen oxides at concentrations of about a few ppm. Conductance measurements have been done in a constant flow of inert atmosphere (NJ and in synthetic air containing oxidizing agents (NO? and NO,) to study directly adsorbed NOz and NO and their reaction with chemisorbed oxygen. The results are discussed in terms of surface reactions. KeywordF:Gas sensors; Oxidizingagents; Tin oxide
1. Introduction For some semiconductor oxides the electrical conductivity is sensitive to changes in the composition of the environmental gas. The most frequently reported result is the sample conductivity as a function of the oxygen pressure at fixed temperature. This phenomenon is not unique for oxygen, and similar results occur for other strong oxidizing agent such as NO or NOz; the oxide films exhibit a decrease of conductance when exposed to oxidizing atmospheres [1,2]. Gas semiconductor sensors base their operation principle on reactions of gas molecules with O,- and Oions previously adsorbed onto sensor surfaces [3]. In this investigation the adsorption-desorption behaviour of 02, NO*, NO, and NO on the SnO, surface is studied as function of the surrounding atmosphere. It is well known that strongly electrophilic gases like NO, and NO, can cause large conductivity variations in SnO, doped with group III elements (Al, In) [4]. Platinum was chosen to compare its effect with that of the trivalent additives. 2. Experimental The films were prepared by reactive sputtering (5% 0, in Ar) from an SnO, target and were deposited on * Corresponding author. 0925400595/$09.50 0 1995 Elsevier Science S.A. All rights reserved SSDI 09254005(94)01407-9
a heated alumina substrate. Dopants were introduced as an intermediate dotted layer by sputtering in argon atmosphere from metallic targets (Pt, In and Al). Gold electrodes were also deposited on the film by sputtering. All sensors had the same thickness, 3000 A, and were submitted to the same thermal treatment, after which the sensors were tested in synthetic air or nitrogen.
3. Results The results of chemisorption of oxidant substances is a decrease in the electron concentration in the conduction band (near the surface); thus the surface conductance is reduced. These effects were observed in the absorption of 0, and nitrogen oxides on SnO,. 3.1. Experiments in synthetic air atmosphere The NO, and NO, responses of all sensors are similar: the increase of resistance in the presence of these gases shows that they present an oxidant behaviour on the tin-oxide surface. The NO, response is higher than that of the NO, sensor; Fig. 1 shows the response curves of the sensors at constant temperature in the presence of 2 ppm NO, and 2 ppm NO,. The sensors present a fast response but their recuperation is very slow; as the temperature increases
513
I. Sayago et al. I Sensors and Actuators B 24-25 (1995) 512-515 +7.BBB+m .(,,,,
R phms]
sensitivities, as shown in Fig. 4. As the temperature increases the sensitivity increases up to a maximum value at 525 K for undoped and Pt-doped sensors; samples doped with In and Al present optimum temperatures below 42.5 and 475 K, respectively, as shown in Fig. 4. The relatively sensitivity is defined as
ah
,,(,(,,,
.‘.‘-..‘..
:“2 pprnNO2
-RairYR*irllOO s =[@NOz NOz (,,,,,,, I,. ._ .._.. :.
.
.
!,,.
‘..
ZppmMo,
..’
et.BBB+m lB . Bm*m
. ._
.. . .. .... _.._............... l.SBE+BZ 3.2. Experiments in nitrogen atmosphere
t [minutes)
l
Fig. 1. Resistance variation with time at 525 K.
,:A:..-
575K
“‘5~~K... . 1:: .“’ ,75K ,;/;::
::::::,,.::
..I
“”
.::,*I/
II...
,,./:,ii/::i,,::::::;;
.,.,__, ,..,(,,,,,_ ‘,
The experiments have been carried out in differents ways: (i) the films were stabilized in nitrogen without being tested previously in air atmosphere; (ii) after the experiments in air, the films were stabilized in nitrogen; (iii) after the experiments in air, the sensors were exposed to nitrogen for one or two hours. In the first case (Fig. 5) the increase of resistance begins with detection; after that the resistance decreases, tending towards a constant value, and in the recovery process the final resistance is always smaller than the initial value. In the successive detections, the modifications in resistance are smaller until there is no detection’ of NO. Fig. 5 shows the resistance variation
..,..,..,,,~,
“““.
.....,:
:...:
::
..,,,,,.... ,,..,...
425 K
Fig. 2. Resistance variation with time at different temperatures 2 ppm NO=.
8 lagR
for
Ah
300K
, ,:..I::.‘. .. ,‘. ‘. ‘. .
575K 526K “‘~75L ..~ 425 K I
+td&30
: ,’
F”‘...,
1;.
”
‘K.,
.Q::::::,;;:‘. ,,.:.il:l:::..::.:j,,ll;j:;:I:,, 1;’ ..’ ‘... ‘.‘... .. .. ,, ,,,, ,. ,’
i675 T (K)
snoz
t
Fig. 4. Sensitivity variation with respect to temperature. I
(minutes]
+9.BBa+o1
Fig. 3. Resistance variation with time at different temperatures 2 ppm NO*.
for
the recuperation time decreases remarkably as shown in Fig. 2, the resistance variation with time at different temperatures when the sensor is submitted to a 2 ppm NO, atmosphere in air for 10 min at different temperatures. When the tensors are exposed to NOZ, the response as a function of temperature is similar (Fig. 3). The sensitivity is considerably increased at low temperatures in samples doped with In and Al, the undoped sensors and sensors doped with Pt present similar
,i i\ ‘\.._.__d’
NT+ 4.mPM +E .aBI+en
1OppmNO * N1 --jG--NN2--+
t [minutes]
Fig. 5. Response curve for SnO,.
l .65DaZ
I. Sayago et al. / Sensors
514
and Actuators B 24-25
with time at ambient temperature when the SnOz sensor is submitted to 10 ppm of NO. In the second case (Fig. 6) the results obtained are similar to those of the first case, though the modtications of the resistance depend on the sensor temperature. Fig. 6 shows the resistance variation with time for SnO, sensors doped with Al at 453 K in the presence of 10 ppm of NO. In the third case (Fig. 7) a decrease of resistance is observed in presence of nitrogen; the recovery process of oxygen desorption is accelerated with an increase
525 K
Fig. 6. Response
El
curves for: (a) SnO+U-910~;
(1995) 512-515
in temperature. The response in nitrogen atmosphere is considerable, but always lower than in air, and at 625 K the detection is similar to that of the sensors stabilized in nitrogen atmosphere. Fig. 7(a) and (b) shows the response for an aluminium-doped sensor at different temperatures in the presence of different concentrations. The response of a sensor doped with In and Al is always higher than that of the undoped and Pt-doped sensors. This is because the trivalent additives favour the adsorption of oxygen ions onto the surface.
4. Proposed mechanism The NO and NOz can be adsorbed on tin oxide in three different states: two nitrosyl types (NO-, NO+) and a nitrito type (NO,-). The strength of the Sn-NO+ bond is generally stronger than that of the Sn-NO-, therefore the nitrogen oxide tends to be adsorbed as NO’ and NO,-. The NO can be adsorbed on the SnO, or can react with the oxygen ions previously adsorbed (at low temperatures the prevailing ionosorbed species is O,-; upon increasing the temperature O- is adsorbed) 161. The surface reactions that can explain the variation of resistance in an air atmosphere are
(b) Sn9.
I
f
NO(g) -
NO+ +e-
NO(g) +02- +e- --+ N02- +O/
Gppm NOx / 4-i :
;y
NO(g) + 0- -
The NO+ ion can interact with previously adsorbed O,- or O- according to the reactions
'i, 'I-_
:-N-
'-
2
NOz-
~PP~N%
NO+ +O,- +2.-- NO+ +0-
-
NO,- +O-
(NO&
Simultaneously, the main part of NO gas in the presence of the oxygen of the air tends to oxidize into NO?. This NO, interacts with the adsorbed oxygen or is adsorbed onto the sensor surface; the surface reactions are NO*(g) +e- -
NO,-
NOz(g) + O,- + 2e- NO,(g)+O‘.
z
4
6 PP~ Wt
‘\._____,---,_.~,.
.__ Ah
Atr-
+lt.La
~~_JF-------.-.
t [minutes]
+3.3m+Ez
lb1
Fig. 7. Response (b) 575 K
cwves for SnOrALSnOz
sensors at (a) 523 K and
-
NOZ- + 20-
NO+ +20-
In nitrogen atmosphere the reactions are similar; the desorption of NO, with the consequent oxygen-ion consumption leads to irreversible processes. This explains why in a nitrogen atmosphere the sensors are initially sensitive to NO. In any case, the participating adsorbed ions are NOzor NO’, which are desorbed as NO,. The adsorption processes are fast but the reactions are slow since there
I. Sayago Edal. I Senws and Actuntm B 24-25 (1995) 512-515
are multiple processes (between the gas to be detected and the adsorbed oxygen species or between the different adsorbed species).
5. conclusions
The different species detected (NO, NO2 and NO,) are oxidants and their oxidant behaviours decrease when the temperature rises. The effect of trivalent additives (In and Al) is an increase of sensitivity because they favour the adsorption of oxygen onto the surface. Among the tested dopants, the most suitable are In and Al. For operation at low temperature (T<475 K) the In-doped sensor could only be used as an alarm device. due to its slow recuperation time. Aluminium could be used as a dopant in sensors operating in the temperature interval 475-525 K.
515
References PI J. Gutierrez, L. A&s, M.C. HorriIlo, I. Sayago, J. G&no and
PI
131 141
[51
[61
J.A. Agapito, NO, tin dioxide sensors activities, as a function of doped materials and temperature, Sen.sor~and Achwtom B, 15-16 (1993) 354-356. I. Sayago, Studio de1 comportamiento de1 6xido de estario coma material sensor para la detecci6n de me&as gaseosas contaminantes, 77wi.q Universidad Complutense de Madrid (1993). S.C. Chang, Thin film semiconductor NO, sensor, IEEE 7’hww. Eiectmn Devices, ED-26 (1979) 1875-1880. G. Sberveglieri, S. Groppelli,P. Nelli, V. Lantto, H. Torvela, P. Romppainen and S. Leppavuori, Response to nitric oxide of thin and thick Sn02 fihns containing trivalent additives, Sensors and Actuators, Bl (1990) 7W2. J. Tamaki, M. Nagaishi, Y. Teraoka, N. MIura, N. Yamazoe, K. Moriya and Y. Nakamura, Adsorption behavior of CO and interfering gases on SnO,, Swjbce SC& 221 (1989) 18>196. S.C. Chang, Oxygen chemisorption on thin oxide: correlation between electrical conductivity and EPR measurement, Surface Sci., 86 (1979) 335-344.
The interaction of different oxidizing agents on doped tin oxide I. Sayago a~*,J.
Gutihrez a, L. Ads a, J.I. Robla a, M.C. Horrillo ‘, J. Getino a, J.A. Agapito b
’ Laboratoriode Sensors, CSIC, Semmo, 144, 28006~Madtid,Spain bDepartamentode Eiechdnica, Facultad CienciasFGxs, UnivwsidadComplutense,Ciudad Universitaria, 28040.Madrid Spain
Abstract The interaction of NO*, NO= and O2 on an SnO, thin-film surface is studied. Experiments have been carried out for different tin-oxide films, undoped and doped with Pt, In and Al, prepared by reactive sputtering (r.f.) on alumina substrates. These films should allow the development of detectors for nitrogen oxides at concentrations of about a few ppm. Conductance measurements have been done in a constant flow of inert atmosphere (NJ and in synthetic air containing oxidizing agents (NO? and NO,) to study directly adsorbed NOz and NO and their reaction with chemisorbed oxygen. The results are discussed in terms of surface reactions. KeywordF:Gas sensors; Oxidizingagents; Tin oxide
1. Introduction For some semiconductor oxides the electrical conductivity is sensitive to changes in the composition of the environmental gas. The most frequently reported result is the sample conductivity as a function of the oxygen pressure at fixed temperature. This phenomenon is not unique for oxygen, and similar results occur for other strong oxidizing agent such as NO or NOz; the oxide films exhibit a decrease of conductance when exposed to oxidizing atmospheres [1,2]. Gas semiconductor sensors base their operation principle on reactions of gas molecules with O,- and Oions previously adsorbed onto sensor surfaces [3]. In this investigation the adsorption-desorption behaviour of 02, NO*, NO, and NO on the SnO, surface is studied as function of the surrounding atmosphere. It is well known that strongly electrophilic gases like NO, and NO, can cause large conductivity variations in SnO, doped with group III elements (Al, In) [4]. Platinum was chosen to compare its effect with that of the trivalent additives. 2. Experimental The films were prepared by reactive sputtering (5% 0, in Ar) from an SnO, target and were deposited on * Corresponding author. 0925400595/$09.50 0 1995 Elsevier Science S.A. All rights reserved SSDI 09254005(94)01407-9
a heated alumina substrate. Dopants were introduced as an intermediate dotted layer by sputtering in argon atmosphere from metallic targets (Pt, In and Al). Gold electrodes were also deposited on the film by sputtering. All sensors had the same thickness, 3000 A, and were submitted to the same thermal treatment, after which the sensors were tested in synthetic air or nitrogen.
3. Results The results of chemisorption of oxidant substances is a decrease in the electron concentration in the conduction band (near the surface); thus the surface conductance is reduced. These effects were observed in the absorption of 0, and nitrogen oxides on SnO,. 3.1. Experiments in synthetic air atmosphere The NO, and NO, responses of all sensors are similar: the increase of resistance in the presence of these gases shows that they present an oxidant behaviour on the tin-oxide surface. The NO, response is higher than that of the NO, sensor; Fig. 1 shows the response curves of the sensors at constant temperature in the presence of 2 ppm NO, and 2 ppm NO,. The sensors present a fast response but their recuperation is very slow; as the temperature increases
513
I. Sayago et al. I Sensors and Actuators B 24-25 (1995) 512-515 +7.BBB+m .(,,,,
R phms]
sensitivities, as shown in Fig. 4. As the temperature increases the sensitivity increases up to a maximum value at 525 K for undoped and Pt-doped sensors; samples doped with In and Al present optimum temperatures below 42.5 and 475 K, respectively, as shown in Fig. 4. The relatively sensitivity is defined as
ah
,,(,(,,,
.‘.‘-..‘..
:“2 pprnNO2
-RairYR*irllOO s =[@NOz NOz (,,,,,,, I,. ._ .._.. :.
.
.
!,,.
‘..
ZppmMo,
..’
et.BBB+m lB . Bm*m
. ._
.. . .. .... _.._............... l.SBE+BZ 3.2. Experiments in nitrogen atmosphere
t [minutes)
l
Fig. 1. Resistance variation with time at 525 K.
,:A:..-
575K
“‘5~~K... . 1:: .“’ ,75K ,;/;::
::::::,,.::
..I
“”
.::,*I/
II...
,,./:,ii/::i,,::::::;;
.,.,__, ,..,(,,,,,_ ‘,
The experiments have been carried out in differents ways: (i) the films were stabilized in nitrogen without being tested previously in air atmosphere; (ii) after the experiments in air, the films were stabilized in nitrogen; (iii) after the experiments in air, the sensors were exposed to nitrogen for one or two hours. In the first case (Fig. 5) the increase of resistance begins with detection; after that the resistance decreases, tending towards a constant value, and in the recovery process the final resistance is always smaller than the initial value. In the successive detections, the modifications in resistance are smaller until there is no detection’ of NO. Fig. 5 shows the resistance variation
..,..,..,,,~,
“““.
.....,:
:...:
::
..,,,,,.... ,,..,...
425 K
Fig. 2. Resistance variation with time at different temperatures 2 ppm NO=.
8 lagR
for
Ah
300K
, ,:..I::.‘. .. ,‘. ‘. ‘. .
575K 526K “‘~75L ..~ 425 K I
+td&30
: ,’
F”‘...,
1;.
”
‘K.,
.Q::::::,;;:‘. ,,.:.il:l:::..::.:j,,ll;j:;:I:,, 1;’ ..’ ‘... ‘.‘... .. .. ,, ,,,, ,. ,’
i675 T (K)
snoz
t
Fig. 4. Sensitivity variation with respect to temperature. I
(minutes]
+9.BBa+o1
Fig. 3. Resistance variation with time at different temperatures 2 ppm NO*.
for
the recuperation time decreases remarkably as shown in Fig. 2, the resistance variation with time at different temperatures when the sensor is submitted to a 2 ppm NO, atmosphere in air for 10 min at different temperatures. When the tensors are exposed to NOZ, the response as a function of temperature is similar (Fig. 3). The sensitivity is considerably increased at low temperatures in samples doped with In and Al, the undoped sensors and sensors doped with Pt present similar
,i i\ ‘\.._.__d’
NT+ 4.mPM +E .aBI+en
1OppmNO * N1 --jG--NN2--+
t [minutes]
Fig. 5. Response curve for SnO,.
l .65DaZ
I. Sayago et al. / Sensors
514
and Actuators B 24-25
with time at ambient temperature when the SnOz sensor is submitted to 10 ppm of NO. In the second case (Fig. 6) the results obtained are similar to those of the first case, though the modtications of the resistance depend on the sensor temperature. Fig. 6 shows the resistance variation with time for SnO, sensors doped with Al at 453 K in the presence of 10 ppm of NO. In the third case (Fig. 7) a decrease of resistance is observed in presence of nitrogen; the recovery process of oxygen desorption is accelerated with an increase
525 K
Fig. 6. Response
El
curves for: (a) SnO+U-910~;
(1995) 512-515
in temperature. The response in nitrogen atmosphere is considerable, but always lower than in air, and at 625 K the detection is similar to that of the sensors stabilized in nitrogen atmosphere. Fig. 7(a) and (b) shows the response for an aluminium-doped sensor at different temperatures in the presence of different concentrations. The response of a sensor doped with In and Al is always higher than that of the undoped and Pt-doped sensors. This is because the trivalent additives favour the adsorption of oxygen ions onto the surface.
4. Proposed mechanism The NO and NOz can be adsorbed on tin oxide in three different states: two nitrosyl types (NO-, NO+) and a nitrito type (NO,-). The strength of the Sn-NO+ bond is generally stronger than that of the Sn-NO-, therefore the nitrogen oxide tends to be adsorbed as NO’ and NO,-. The NO can be adsorbed on the SnO, or can react with the oxygen ions previously adsorbed (at low temperatures the prevailing ionosorbed species is O,-; upon increasing the temperature O- is adsorbed) 161. The surface reactions that can explain the variation of resistance in an air atmosphere are
(b) Sn9.
I
f
NO(g) -
NO+ +e-
NO(g) +02- +e- --+ N02- +O/
Gppm NOx / 4-i :
;y
NO(g) + 0- -
The NO+ ion can interact with previously adsorbed O,- or O- according to the reactions
'i, 'I-_
:-N-
'-
2
NOz-
~PP~N%
NO+ +O,- +2.-- NO+ +0-
-
NO,- +O-
(NO&
Simultaneously, the main part of NO gas in the presence of the oxygen of the air tends to oxidize into NO?. This NO, interacts with the adsorbed oxygen or is adsorbed onto the sensor surface; the surface reactions are NO*(g) +e- -
NO,-
NOz(g) + O,- + 2e- NO,(g)+O‘.
z
4
6 PP~ Wt
‘\._____,---,_.~,.
.__ Ah
Atr-
+lt.La
~~_JF-------.-.
t [minutes]
+3.3m+Ez
lb1
Fig. 7. Response (b) 575 K
cwves for SnOrALSnOz
sensors at (a) 523 K and
-
NOZ- + 20-
NO+ +20-
In nitrogen atmosphere the reactions are similar; the desorption of NO, with the consequent oxygen-ion consumption leads to irreversible processes. This explains why in a nitrogen atmosphere the sensors are initially sensitive to NO. In any case, the participating adsorbed ions are NOzor NO’, which are desorbed as NO,. The adsorption processes are fast but the reactions are slow since there
I. Sayago Edal. I Senws and Actuntm B 24-25 (1995) 512-515
are multiple processes (between the gas to be detected and the adsorbed oxygen species or between the different adsorbed species).
5. conclusions
The different species detected (NO, NO2 and NO,) are oxidants and their oxidant behaviours decrease when the temperature rises. The effect of trivalent additives (In and Al) is an increase of sensitivity because they favour the adsorption of oxygen onto the surface. Among the tested dopants, the most suitable are In and Al. For operation at low temperature (T<475 K) the In-doped sensor could only be used as an alarm device. due to its slow recuperation time. Aluminium could be used as a dopant in sensors operating in the temperature interval 475-525 K.
515
References PI J. Gutierrez, L. A&s, M.C. HorriIlo, I. Sayago, J. G&no and
PI
131 141
[51
[61
J.A. Agapito, NO, tin dioxide sensors activities, as a function of doped materials and temperature, Sen.sor~and Achwtom B, 15-16 (1993) 354-356. I. Sayago, Studio de1 comportamiento de1 6xido de estario coma material sensor para la detecci6n de me&as gaseosas contaminantes, 77wi.q Universidad Complutense de Madrid (1993). S.C. Chang, Thin film semiconductor NO, sensor, IEEE 7’hww. Eiectmn Devices, ED-26 (1979) 1875-1880. G. Sberveglieri, S. Groppelli,P. Nelli, V. Lantto, H. Torvela, P. Romppainen and S. Leppavuori, Response to nitric oxide of thin and thick Sn02 fihns containing trivalent additives, Sensors and Actuators, Bl (1990) 7W2. J. Tamaki, M. Nagaishi, Y. Teraoka, N. MIura, N. Yamazoe, K. Moriya and Y. Nakamura, Adsorption behavior of CO and interfering gases on SnO,, Swjbce SC& 221 (1989) 18>196. S.C. Chang, Oxygen chemisorption on thin oxide: correlation between electrical conductivity and EPR measurement, Surface Sci., 86 (1979) 335-344.