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Platinum-titania oxygen sensors and their sensing mechanisms
Senson and Aciuators B, 13-14 (1993) 492-494
492
Platinum-titania oxygen sensors and their sensing mechanisms Yulong Xu, Kui Yao, Xiaohua Zhou and Quanxi Cao Deparbnmtof Technical Physics,Xidian Univmi~, Wan 710071 (China)
Tbe combustion processes of fuel in the engines of automobiles can be evaluated by using the oxygen sensors based on TiO, semiconducting ceramics, so that the fuel consumption can be cut down and the environmental pollution can be reduced [l]. In this research area, a lot of work has been done and marked achievements have been made [2], but unsolved problems are not rare. People are developing new processes, particularly on sensing mechanism in order to improve the performance of the sensors.
Sensor fabrkation Tbe superfine and highly-pure powder of TiO, is made from TiCl, raw material through the liquid-phase codeposition method. Metal platinum is doped as catalyst at the first time to the TiO, powder by the reduction of cbloroplatinic acid in liquid phase. Ceramic sintering body and thick-film oxygen sensors are fabricated by ceramic sintering processes and by screen printing tbickfilm processes, respectively. Tbe second platinum catalyzing of the sensors is realized by tbermodecomposition of cbloroplatinic acid at low temperature. The sensors undergone the first time platinum catalyzing are called type 1 sensors and those undergone two times platinum catalyzing are called type 2 sensors.
1
0
1
CO / N,(%)
2 0, / N,(%)
Fig. 1. Resistance vs. atmosphere for type 1 senwr at different temperatures.
10
z a
9
4
8
*
7
F
24Oc 40072
5501:
-z!E6
75oc
5
-4
-
1
CO/N,(%)
0
I
2
&/N,%
Performance and microanalyses of sensors
Fig. 2. Resistance vs. atmosphere for type 2 seasor at different temperatures.
Tbe sensors are tested in a quartz glass tube whose temperature and atmosphere can be controlled. The typical testing results of the sintering body sensors are shown iu Figs. 1 and 2. Tbe typical testing results for thick-film sensors are similar to Figs. 1 and 2. Tbe response time of type 1 sensors is generally between 400 and 900 ms, the recuperation time of type 1 sensors is generally between several seconds and several tens of seconds. Tbe response time of type 2 sensors is generally between 200 and 800 ms. Tbe recuperation time of type 2 sensors is between 600 ms and 4 s. It
is obvious that type 2 sensors respond faster than type 1 sensors. Over a wide temperature range (2.S 750 “C), type 2 sensors have a high sensitivity, very rapid response and recuperation. Tbe relations of the sensors’ resistance to temperature are also tested at reductive and oxidative atmosphere, respectively. Figure 3 shows the testing results for type 2 sensors, where curve &, is the log R- 1000/T curve in oxidation atmosphere and curve R, is the log R1000/T curve in reductive atmosphere. From Fig. 3 it is evident that the log R - 1000/T cmves deviate from
0925-4005/93/s6.00
0 1993-Elsevier
Sequoia. All rights reserved
493
0.8
1.2
1.6
2.0
1000/TW-')
Fig. 3.
IogR-IOWT
curves
of type 2 sensor at different at-
a straight line and the resistance temperature coefficient is very small at low temperature. Moreover, the sensitivity at low temperature is much higher than that at high temperature. All of these are different from type 1 sensors. Microanalyses manifest that the main crystalline phase of type 1 and type 2 sensors is TiO, rutile structure. The grain size is less than one pm. The metal platinum content in type 2 sensors is about 0.5 wt.% and most of them are distributed in grain boundaries.
Oxygen sensing mechanisms
buk effect The oxygensensing mechanism of type 1 sensors is
Mechankm of qgen-vacancy
considered to be oxygen-vacancy bulk effect. The mechanism states that a lot of oxygen vacancies exist in TiO, material and lattice oxygen ions can exchange with the oxygen of environmental atmosphere through the diEusion of oxygen ions or atoms at temperatures high enough to maintain the balance between the oxygen of environmental atmosphere, the lattice oxygen and the lattice oxygen vacancies [3]. The exchange rate increases with temperature. According to this mechanism, the sensitivity of the sensors at high temperature should be higher than that at low temperature; the response and recuperation rate at high temperature should be faster than that at low temperature, and the log R- 1OWT curve should be a straight line. It is evident that this mechanism can not explain Fig. 3.
Mechanismof Schottky-bark fleet Because the work function of metal platinum (5.6 eV) is larger than the difference between the vacuum energy level and the Fermi level of TiO, (4.6-5.5 eV),
the Schottky barrier can form between metal platinum and semiconducting ceramic of TiOz when they contact with each other [4]. The adsorption of oxygen in platinum-TiO, interface can change the height and the width of Schottky barrier so that the resistance of the sensor is changed. This is the principle of Schottkybarrier mechanism. Because the doped platinum at the first time underwent temperatures above 1100 “C, the platinum was oxidated into PtOz and PtO. The Pt4’ and PP’ can ditruse into the TiO, lattice easily due to the approximate sixes of Pt4+, Pf+ and Ti4’ and temperatures high enough so that the Pt-TiO, Schottky-barrier mechanism is not suited to type 1 sensors. As regards type 2 sensors, the thermodecomposition temperature of chloroplatinic acid is so low that the platinum can not be oxidated and does not diffuse easily into the TiOz lattice, So the Schottky barrier exists at Pt-TiOz interfaces [4]. The oxygen-sensing mechanism of Schottky barrier is a grain-boundary effect, therefore it does not require solid diffusion. As a result the sensors has a high speed response and high sensitivity even at very low temperature. The logR-1000/T curves can be explained by Schottky-barrier mechanism satisfactorily. When the temperature is low, the tunnel current dominates in the sensor, particularly in the reductive atmosphere because the electron density of TiO, near the Pt-TiO* interface is so high that the Schottky barrier is very thin and the quantum-tunnel effect dominates more. The resistance-temperature coefficient of the sensor is very small by virtue of the tunnel effect being mdependent of temperature essentially. With increasing temperature, the current striding across the Schottky barrier increases markedly. It is given by the following formula:
J=AT’~ exp( -B/W) where A and B are constants, KBis Boltxmann’s constant and T is the temperature of the sensor. As a result, the resistance temperature coe5cient of the sensor increases. With still increasing temperature, the electron density increases markedly due to the intrinsic excitation and the Fermi level moves towards the middle of forbidden band. This causes the height of Pt-TiO, barrier to decrease and the role of Schottky barrier to be abated. But with the ion’s diffusion being enhanced at higher temperature, the role of oxygen-vacancy bulk effect is enhanced. Our own experiments show that when T<450 “C, the oxygen-sensing mechanism of Schottky barrier dominates, while the oxygen-vacancy bulk effect dominates when T> 550 “C. In the transition region (450 “C < T< 550 “C), both of two mechanisms
494
play a role. These are the reasons that the log R1000/Tcurves deviates from a straight line.
Conclusions The second platinum catalyzingof sensors based on superfineTiOz through thermal decompositionof chloroplatinic acid at low temperature, can improve the performance of the sensors markedly,in particular, the low temperature performance. The oxygen-sensingmechanism, which is the combination of Pt-TiOz interfaces Shot&y-barrier mechanism and oxygen-vacancybulk effect mechanism,can be used to explain the experimental results of type 2 oxygen sensors satisfactorily. The conclusionsof this paper are valuableto improve and stabilizethe performanceof Pt-TiO,oxygen sensors.
Acknowledgements
The authors wishto thank associateProfs Xilin Yan, Jiheng Qiu and Engineer Jinxiu Gao for their excellent technical assistance.The authors are also grateful to Mr Ai jun Ding for his assistance to prepare this manuscript. References 1 T. Y. Tien, H. L. Stadler, F,. F. Gibbons and P. J. Zacmanidis, TiOz as an air-to-fuel ratio sensor for automobile exhausts, 1. Am. Sot. Cemm. Bull, 54 (1975) 28CM85. 2 T. Takeuchi, Oxygen sensors, .%n.ws and Actuators, 14 (1988) w-121. 3 U. Kiner, K. D. Schierbaum and W. Goepel, Low and high temperature TiOz oxygen sensors, Senrors and Acfuators, Bl (1990) 103-107. 4 K. D. Schierbaum, U. K Kiner, J. F. Geiger and W. Goepel, Scbottky-barrier and conductivity gas sensors based upon Pd/ SnOl and PfliO*, Sensors nnd Actuarors B, 4 (1991) 87-94.
492
Platinum-titania oxygen sensors and their sensing mechanisms Yulong Xu, Kui Yao, Xiaohua Zhou and Quanxi Cao Deparbnmtof Technical Physics,Xidian Univmi~, Wan 710071 (China)
Tbe combustion processes of fuel in the engines of automobiles can be evaluated by using the oxygen sensors based on TiO, semiconducting ceramics, so that the fuel consumption can be cut down and the environmental pollution can be reduced [l]. In this research area, a lot of work has been done and marked achievements have been made [2], but unsolved problems are not rare. People are developing new processes, particularly on sensing mechanism in order to improve the performance of the sensors.
Sensor fabrkation Tbe superfine and highly-pure powder of TiO, is made from TiCl, raw material through the liquid-phase codeposition method. Metal platinum is doped as catalyst at the first time to the TiO, powder by the reduction of cbloroplatinic acid in liquid phase. Ceramic sintering body and thick-film oxygen sensors are fabricated by ceramic sintering processes and by screen printing tbickfilm processes, respectively. Tbe second platinum catalyzing of the sensors is realized by tbermodecomposition of cbloroplatinic acid at low temperature. The sensors undergone the first time platinum catalyzing are called type 1 sensors and those undergone two times platinum catalyzing are called type 2 sensors.
1
0
1
CO / N,(%)
2 0, / N,(%)
Fig. 1. Resistance vs. atmosphere for type 1 senwr at different temperatures.
10
z a
9
4
8
*
7
F
24Oc 40072
5501:
-z!E6
75oc
5
-4
-
1
CO/N,(%)
0
I
2
&/N,%
Performance and microanalyses of sensors
Fig. 2. Resistance vs. atmosphere for type 2 seasor at different temperatures.
Tbe sensors are tested in a quartz glass tube whose temperature and atmosphere can be controlled. The typical testing results of the sintering body sensors are shown iu Figs. 1 and 2. Tbe typical testing results for thick-film sensors are similar to Figs. 1 and 2. Tbe response time of type 1 sensors is generally between 400 and 900 ms, the recuperation time of type 1 sensors is generally between several seconds and several tens of seconds. Tbe response time of type 2 sensors is generally between 200 and 800 ms. Tbe recuperation time of type 2 sensors is between 600 ms and 4 s. It
is obvious that type 2 sensors respond faster than type 1 sensors. Over a wide temperature range (2.S 750 “C), type 2 sensors have a high sensitivity, very rapid response and recuperation. Tbe relations of the sensors’ resistance to temperature are also tested at reductive and oxidative atmosphere, respectively. Figure 3 shows the testing results for type 2 sensors, where curve &, is the log R- 1000/T curve in oxidation atmosphere and curve R, is the log R1000/T curve in reductive atmosphere. From Fig. 3 it is evident that the log R - 1000/T cmves deviate from
0925-4005/93/s6.00
0 1993-Elsevier
Sequoia. All rights reserved
493
0.8
1.2
1.6
2.0
1000/TW-')
Fig. 3.
IogR-IOWT
curves
of type 2 sensor at different at-
a straight line and the resistance temperature coefficient is very small at low temperature. Moreover, the sensitivity at low temperature is much higher than that at high temperature. All of these are different from type 1 sensors. Microanalyses manifest that the main crystalline phase of type 1 and type 2 sensors is TiO, rutile structure. The grain size is less than one pm. The metal platinum content in type 2 sensors is about 0.5 wt.% and most of them are distributed in grain boundaries.
Oxygen sensing mechanisms
buk effect The oxygensensing mechanism of type 1 sensors is
Mechankm of qgen-vacancy
considered to be oxygen-vacancy bulk effect. The mechanism states that a lot of oxygen vacancies exist in TiO, material and lattice oxygen ions can exchange with the oxygen of environmental atmosphere through the diEusion of oxygen ions or atoms at temperatures high enough to maintain the balance between the oxygen of environmental atmosphere, the lattice oxygen and the lattice oxygen vacancies [3]. The exchange rate increases with temperature. According to this mechanism, the sensitivity of the sensors at high temperature should be higher than that at low temperature; the response and recuperation rate at high temperature should be faster than that at low temperature, and the log R- 1OWT curve should be a straight line. It is evident that this mechanism can not explain Fig. 3.
Mechanismof Schottky-bark fleet Because the work function of metal platinum (5.6 eV) is larger than the difference between the vacuum energy level and the Fermi level of TiO, (4.6-5.5 eV),
the Schottky barrier can form between metal platinum and semiconducting ceramic of TiOz when they contact with each other [4]. The adsorption of oxygen in platinum-TiO, interface can change the height and the width of Schottky barrier so that the resistance of the sensor is changed. This is the principle of Schottkybarrier mechanism. Because the doped platinum at the first time underwent temperatures above 1100 “C, the platinum was oxidated into PtOz and PtO. The Pt4’ and PP’ can ditruse into the TiO, lattice easily due to the approximate sixes of Pt4+, Pf+ and Ti4’ and temperatures high enough so that the Pt-TiO, Schottky-barrier mechanism is not suited to type 1 sensors. As regards type 2 sensors, the thermodecomposition temperature of chloroplatinic acid is so low that the platinum can not be oxidated and does not diffuse easily into the TiOz lattice, So the Schottky barrier exists at Pt-TiOz interfaces [4]. The oxygen-sensing mechanism of Schottky barrier is a grain-boundary effect, therefore it does not require solid diffusion. As a result the sensors has a high speed response and high sensitivity even at very low temperature. The logR-1000/T curves can be explained by Schottky-barrier mechanism satisfactorily. When the temperature is low, the tunnel current dominates in the sensor, particularly in the reductive atmosphere because the electron density of TiO, near the Pt-TiO* interface is so high that the Schottky barrier is very thin and the quantum-tunnel effect dominates more. The resistance-temperature coefficient of the sensor is very small by virtue of the tunnel effect being mdependent of temperature essentially. With increasing temperature, the current striding across the Schottky barrier increases markedly. It is given by the following formula:
J=AT’~ exp( -B/W) where A and B are constants, KBis Boltxmann’s constant and T is the temperature of the sensor. As a result, the resistance temperature coe5cient of the sensor increases. With still increasing temperature, the electron density increases markedly due to the intrinsic excitation and the Fermi level moves towards the middle of forbidden band. This causes the height of Pt-TiO, barrier to decrease and the role of Schottky barrier to be abated. But with the ion’s diffusion being enhanced at higher temperature, the role of oxygen-vacancy bulk effect is enhanced. Our own experiments show that when T<450 “C, the oxygen-sensing mechanism of Schottky barrier dominates, while the oxygen-vacancy bulk effect dominates when T> 550 “C. In the transition region (450 “C < T< 550 “C), both of two mechanisms
494
play a role. These are the reasons that the log R1000/Tcurves deviates from a straight line.
Conclusions The second platinum catalyzingof sensors based on superfineTiOz through thermal decompositionof chloroplatinic acid at low temperature, can improve the performance of the sensors markedly,in particular, the low temperature performance. The oxygen-sensingmechanism, which is the combination of Pt-TiOz interfaces Shot&y-barrier mechanism and oxygen-vacancybulk effect mechanism,can be used to explain the experimental results of type 2 oxygen sensors satisfactorily. The conclusionsof this paper are valuableto improve and stabilizethe performanceof Pt-TiO,oxygen sensors.
Acknowledgements
The authors wishto thank associateProfs Xilin Yan, Jiheng Qiu and Engineer Jinxiu Gao for their excellent technical assistance.The authors are also grateful to Mr Ai jun Ding for his assistance to prepare this manuscript. References 1 T. Y. Tien, H. L. Stadler, F,. F. Gibbons and P. J. Zacmanidis, TiOz as an air-to-fuel ratio sensor for automobile exhausts, 1. Am. Sot. Cemm. Bull, 54 (1975) 28CM85. 2 T. Takeuchi, Oxygen sensors, .%n.ws and Actuators, 14 (1988) w-121. 3 U. Kiner, K. D. Schierbaum and W. Goepel, Low and high temperature TiOz oxygen sensors, Senrors and Acfuators, Bl (1990) 103-107. 4 K. D. Schierbaum, U. K Kiner, J. F. Geiger and W. Goepel, Scbottky-barrier and conductivity gas sensors based upon Pd/ SnOl and PfliO*, Sensors nnd Actuarors B, 4 (1991) 87-94.