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Effect of Nb metal ion in TiO2 oxygen gas sensor
surface science ELSEVIER
Applied SurfaceScience 92 (1996) 647-650
Effect of Nb metal ion in T i O 2 oxygen gas sensor Rajnish K. Sharma, Mukesh C. Bhatnagar, G.L. Sharma * Thin Film Laboratory, Department of Physics, Indian Institute of Technology, New Delhi 110 016, India
Received 13 December 1994; acceptedfor publication20 June 1995
Abstract
The electrical conductivity of TiO 2 material is dependent upon the operating temperature and oxygen partial pressure. This characteristic of the material has been exploited for use as an oxygen gas sensor. The effect of bib incorporation in TiO2 has also been investigated. The sensor response and recovery time as a function of Nb concentration have also been studied.
I. I n t r o d u c t i o n
Solid state gas sensors are frequently used in various fields of gas detection. Oxygen gas sensors are widely used as monitoring and control devices in various applications in the industry, i.e. automobile and metallurgical industries, environmental and biological control processes, etc. For this purpose, various metal oxides have been used to design oxygen sensors operating at high temperatures, as well as room temperature. The effect of the oxygen concentration on the resistivity of the oxide was observed by Shimizu et al. [1,2] who suggested that if n-type metal-oxide semiconductors are exposed to 0 2, their resistivity increases. The phenomena is described in the Kroger-Vink notation as 1
O * ,a, V5 + ~O 2 + 2e,
(l)
where V5 are charged oxygen vacancies and O* are the lattice oxygen sites. As the oxygen partial pressure decreases, the oxygen ions migrate to the sur-
* Corresponding author. Fax: +91 11 6862037; e-mail: [email protected].
face, creating additional oxygen defects in the bulk. As the number of the vacancies increases, more electrons are donated and the resistivity of the material decreases [3]. The addition of a noble metal increases the sensitivity and selectivity of the oxygen sensor [4] and hence is advantageous for vehicle applications as the low impedance makes it simpler to design the electronic circuit and improve the fast activation of the sensor [5]. Also, titania sensors have superior durability against lead poisoning as compared to zirconia sensors [6]. Niobium as a dopant material has so far received very little attention, therefore, the present study investigates the structural, electrical and oxygen sensing properties of Nb doped titania pellets.
2. E x p e r i m e n t a l
TiO 2 (99.99% purity) powders containing 0.2 and 0.4 wt% Nb were used. These powders were properly grounded and heated at 1300°C for four hours. After cooling, the material was again grounded and converted into a disk. Electrical connections were
0169-4332/96/$15.00 © 1996 Elsevier Science B.V. All fights reserved SSD! 0169-4332(95)003 11-8
648
R.K. Sharma et a l . / Applied Surface Science 92 (1996) 647-650
made to the disk through a pair of Pt wires and heated at the same temperature for two hours. The structure was studied by X-ray diffraction using a 12 kW rotating anode Rigaku RB-R0200 system. The resistivity of these pellets was measured as a function of oxygen partial pressure. A mixture of Ar and 0 2 has been used for varying the partial oxygen pressure. The gas pressures have been regulated by mass flow controllers (MFC). The response time and the recovery time of the sensors have been recorded on an X - t recorder at different operating temperatures for a particular oxygen concentration.
6.0
"-
(a)
(b)
(c)
g
(a)
40
650 °C
/
2
"
b
4.0
c~3.0
q. 2.0
1.0
-5.0
I -4.o
I -3.o
I -2.0
I -1.0
-0.o
Po2 (Torr)
The X-ray diffraction data of all the samples show the existence of the rutile phase only, as shown in Fig. 1. The other two polymorphic forms anatase and brookite are metastable and transform exothermally
20
/
600 °C
5.0
0.0
3. Results and discussion
---o--- SO0 ~:
6O
80
20 (Degree) Fig. 1. XRD patterns of TiO 2 with Nb doping, (a) 0.8 wt%, (b) 0.4 wt%, (c) 0.2 wt%, and (d) 0.0 wt% Nb.
Fig. 2. The ratio P / P o of TiO 2 as a function of oxygen partial pressure at different temperatures.
and irreversibly to the rutile phase at temperatures of ~ 750°C and ~ 1000°C, respectively. However, it has been shown that the substitution of impurity ions of valency greater than four, such as S 6+, Nb 5+ and pS+ reduces the oxygen vacancy concentration and inhibits the transformation [7]. Also large quantities of most impurities raise the transformation temperature. Since the ionic radii of Nb 5+ (0.70,4,) is larger than the ionic crystal radius of 0.68 ,~ for Ti 4+, their incorporation either substitutionaUy or interstitially will introduce stress in the titania lattice. This stress may also hinder the growth of rutile phase crystallites. The sintering temperature in the present study was kept at 1300°C to get only the rutile phase. However, the presence of niobium which inhibits the transformation at 1000°C, prevents the growth of crystallites at higher temperature. This has been observed in the reduction of all the peak intensities in the XRD pattern as the Nb content increases from 0.0 to 0.8 wt%. Fig. 2 shows the ratio of the change in resistivity of TiO 2 material as a function of oxygen partial pressure at different temperatures. Here P0 is the resistivity of the material in the absence of oxygen and 0 is the resistivity in the presence of oxygen. The larger change in the resistivity has been observed at 650°C as compared to lower operating temperatures. Fig. 3 shows the ratio of the change in
R.K. Sharma et al. / Applied Surface Science 92 (1996) 647-650
649
80.0 A : PURE T;Oz ,
---o---
& O0 QC sO0 *c 600 QC
19
-
6SO~C
/ --
60.0
C s 0./,el, Nb ~
/
c
tU tJ
z
TEMPERATURE
a = 0•2 q . Nb O = 0 - E ' / * Nb 100eC
c~ ~;O.C qL
20.0 -
¢
OO.G -5.0
-4.0
-3-0 -1.0 Poz (Torr)
-1.0
-0.0
Fig, 3. The ratio P/Po of TiO 2 as a function of oxygen partial pressure at different temperatures for 0.2 wt% Nb.
0
i
2
4
I
6
t
8
TIRE (rain) Fig. 5. Response and recovery time of pure TiO 2 and Nb doped sensors at an oxygen concentration of 1200 ppm.
resistivity of TiO 2 with 0.2 wt% Nb with different oxygen partial pressures. The measurement has been carried out at 400, 500, 600 and 650°C at the same oxygen partial pressure• Here the maximum change in resistivity has been observed at 400°C. The results indicate that higher sensitivities can be obtained at lower operating temperatures with Nb doping as
1,00 ~2
A A 10--2 Torr • • 10"1 Torr
00 10-& Torr a O 10-3 Torr 300
-/ u
20t
A w
,,- 10¢ •
o
ml
0
100
0
0
I
200
o
I
o
o
o
b
300 400 TIME (Hr$)
I
500
600
Fig. 4. Number of repeated operation cycles of the sensor at 5000C.
compared to pure undoped titanium oxide samples. A comparable result has also been observed in 0.4 wt% Nb doped samples which exhibited a maximum change in resistivity at 450°C. These results show that a lower operating temperature of the sensor can be achieved by doping low Nb concentrations. Also, the maximum change in the resistivity was observed in Nb doped samples. Pure TiO 2 samples show poor sensitivity for oxygen detection because of their low catalytic activity at the optimum operating temperature [8]. The catalytic activities can be increased by the addition of noble metals such as Pd or Pt [9]. The sensitivity can also be increased by the use suitable dopants which increase the electrical conductivity by providing additional charge carriers. As the Nb concentration increases, the resistivity of the samples decreases due to a more enhanced contribution of Nb donor ions. Fig. 4 shows the number of operating cycles carried out on the samples at 500°C for several hours. The material shows no degradation after a number of operating cycles, and is hence found suitable for long term operations. Fig. 5 shows the response time and the recovery time of a pure TiO 2 and of TiO 2 containing 0.2 and 0.4 wt% Nb sensors at 600°C for a fixed oxygen
650
R.K. Sharma et a l . / Applied Surface Science 92 (1996) 647-650
concentration of 1200 ppm. The sensors containing 0.2 and 0.4 wt% Nb show response times of 6.0 and 6.6 s, respectively, whereas pure TiO 2 samples have a response time of 28 s. When the niobium concentration was increased, the response time increased to 36 s. This may be due to the faster movement of the oxygen vacancies TiO 2 samples containing Nb.
4. Conclusions The present study evaluated the effects of Nb doping of titania pellets used for oxygen sensing applications. It established that Nb incorporation prevents the growth of rutile phase crystallites at 1300°C. By increasing the Nb concentration the crystallite size decreases gradually and hence the material loses its sensitivity for oxygen detection. The maximum sensitivity was obtained at 0.2 wt% Nb at an operating temperature of 400°C. The response and recovery times of the Nb incorporated sensor are also shorter than in the pure TiO 2 sensor. Repeated cycles have almost no effect on the sensitivity, hence this sensor is suitable for long time operations.
Acknowledgements The authors acknowledge the financial support given by the CSIR
References [1] Y. Shimizu, Y. Fukuyama, H. Arai and T. Seiyama, Oxygen sensor using pcrvoskite-type oxides: measurements of electrical characteristics, in: Fundamentals and Applications of Chemical Sensors, Eds. D. Schuetzle and R. Hammerle, ACS Syrup. Ser. Vol. 309 (1986) pp. 83-100. [2] P.T. Moselay, Sensors Actuators B 6 (1992) 149. [3] D.S. Howarth and A.L. Micheli, Soc. Automotive Engineers, Inc. 840140 (1985) 1.827-1.833. [4] D. Kohl, Sensors Actuators B 1 (199.1) 158. [5] A. Takami, C-eram. Bull. 67 (1988) 1956. [6] A. Takami, T. Matsuura, T. Sekiya, T. Okawa and Y. Watanabe, Soc. Automotive Engineers, Inc. 850381 (1986) 3.1873.199. [7] M. Kamal Akhtar and S. E. Pratsinis, J. Am. Ceram. Soc. 75 (1992) 3408. [8] H. Arai and Y. Shimizu, in: Fundamentals and Applications of Chemical Sensors, F_As.D. Schuetzle and R. Hammefle, ACS Syrup. Ser. Vol. 309 (1986) pp. 83-98. [9] E.M. Logothetis and WJ. Kaiser, Sensors Actuators Chem. B 4 (1983) 333.
Applied SurfaceScience 92 (1996) 647-650
Effect of Nb metal ion in T i O 2 oxygen gas sensor Rajnish K. Sharma, Mukesh C. Bhatnagar, G.L. Sharma * Thin Film Laboratory, Department of Physics, Indian Institute of Technology, New Delhi 110 016, India
Received 13 December 1994; acceptedfor publication20 June 1995
Abstract
The electrical conductivity of TiO 2 material is dependent upon the operating temperature and oxygen partial pressure. This characteristic of the material has been exploited for use as an oxygen gas sensor. The effect of bib incorporation in TiO2 has also been investigated. The sensor response and recovery time as a function of Nb concentration have also been studied.
I. I n t r o d u c t i o n
Solid state gas sensors are frequently used in various fields of gas detection. Oxygen gas sensors are widely used as monitoring and control devices in various applications in the industry, i.e. automobile and metallurgical industries, environmental and biological control processes, etc. For this purpose, various metal oxides have been used to design oxygen sensors operating at high temperatures, as well as room temperature. The effect of the oxygen concentration on the resistivity of the oxide was observed by Shimizu et al. [1,2] who suggested that if n-type metal-oxide semiconductors are exposed to 0 2, their resistivity increases. The phenomena is described in the Kroger-Vink notation as 1
O * ,a, V5 + ~O 2 + 2e,
(l)
where V5 are charged oxygen vacancies and O* are the lattice oxygen sites. As the oxygen partial pressure decreases, the oxygen ions migrate to the sur-
* Corresponding author. Fax: +91 11 6862037; e-mail: [email protected].
face, creating additional oxygen defects in the bulk. As the number of the vacancies increases, more electrons are donated and the resistivity of the material decreases [3]. The addition of a noble metal increases the sensitivity and selectivity of the oxygen sensor [4] and hence is advantageous for vehicle applications as the low impedance makes it simpler to design the electronic circuit and improve the fast activation of the sensor [5]. Also, titania sensors have superior durability against lead poisoning as compared to zirconia sensors [6]. Niobium as a dopant material has so far received very little attention, therefore, the present study investigates the structural, electrical and oxygen sensing properties of Nb doped titania pellets.
2. E x p e r i m e n t a l
TiO 2 (99.99% purity) powders containing 0.2 and 0.4 wt% Nb were used. These powders were properly grounded and heated at 1300°C for four hours. After cooling, the material was again grounded and converted into a disk. Electrical connections were
0169-4332/96/$15.00 © 1996 Elsevier Science B.V. All fights reserved SSD! 0169-4332(95)003 11-8
648
R.K. Sharma et a l . / Applied Surface Science 92 (1996) 647-650
made to the disk through a pair of Pt wires and heated at the same temperature for two hours. The structure was studied by X-ray diffraction using a 12 kW rotating anode Rigaku RB-R0200 system. The resistivity of these pellets was measured as a function of oxygen partial pressure. A mixture of Ar and 0 2 has been used for varying the partial oxygen pressure. The gas pressures have been regulated by mass flow controllers (MFC). The response time and the recovery time of the sensors have been recorded on an X - t recorder at different operating temperatures for a particular oxygen concentration.
6.0
"-
(a)
(b)
(c)
g
(a)
40
650 °C
/
2
"
b
4.0
c~3.0
q. 2.0
1.0
-5.0
I -4.o
I -3.o
I -2.0
I -1.0
-0.o
Po2 (Torr)
The X-ray diffraction data of all the samples show the existence of the rutile phase only, as shown in Fig. 1. The other two polymorphic forms anatase and brookite are metastable and transform exothermally
20
/
600 °C
5.0
0.0
3. Results and discussion
---o--- SO0 ~:
6O
80
20 (Degree) Fig. 1. XRD patterns of TiO 2 with Nb doping, (a) 0.8 wt%, (b) 0.4 wt%, (c) 0.2 wt%, and (d) 0.0 wt% Nb.
Fig. 2. The ratio P / P o of TiO 2 as a function of oxygen partial pressure at different temperatures.
and irreversibly to the rutile phase at temperatures of ~ 750°C and ~ 1000°C, respectively. However, it has been shown that the substitution of impurity ions of valency greater than four, such as S 6+, Nb 5+ and pS+ reduces the oxygen vacancy concentration and inhibits the transformation [7]. Also large quantities of most impurities raise the transformation temperature. Since the ionic radii of Nb 5+ (0.70,4,) is larger than the ionic crystal radius of 0.68 ,~ for Ti 4+, their incorporation either substitutionaUy or interstitially will introduce stress in the titania lattice. This stress may also hinder the growth of rutile phase crystallites. The sintering temperature in the present study was kept at 1300°C to get only the rutile phase. However, the presence of niobium which inhibits the transformation at 1000°C, prevents the growth of crystallites at higher temperature. This has been observed in the reduction of all the peak intensities in the XRD pattern as the Nb content increases from 0.0 to 0.8 wt%. Fig. 2 shows the ratio of the change in resistivity of TiO 2 material as a function of oxygen partial pressure at different temperatures. Here P0 is the resistivity of the material in the absence of oxygen and 0 is the resistivity in the presence of oxygen. The larger change in the resistivity has been observed at 650°C as compared to lower operating temperatures. Fig. 3 shows the ratio of the change in
R.K. Sharma et al. / Applied Surface Science 92 (1996) 647-650
649
80.0 A : PURE T;Oz ,
---o---
& O0 QC sO0 *c 600 QC
19
-
6SO~C
/ --
60.0
C s 0./,el, Nb ~
/
c
tU tJ
z
TEMPERATURE
a = 0•2 q . Nb O = 0 - E ' / * Nb 100eC
c~ ~;O.C qL
20.0 -
¢
OO.G -5.0
-4.0
-3-0 -1.0 Poz (Torr)
-1.0
-0.0
Fig, 3. The ratio P/Po of TiO 2 as a function of oxygen partial pressure at different temperatures for 0.2 wt% Nb.
0
i
2
4
I
6
t
8
TIRE (rain) Fig. 5. Response and recovery time of pure TiO 2 and Nb doped sensors at an oxygen concentration of 1200 ppm.
resistivity of TiO 2 with 0.2 wt% Nb with different oxygen partial pressures. The measurement has been carried out at 400, 500, 600 and 650°C at the same oxygen partial pressure• Here the maximum change in resistivity has been observed at 400°C. The results indicate that higher sensitivities can be obtained at lower operating temperatures with Nb doping as
1,00 ~2
A A 10--2 Torr • • 10"1 Torr
00 10-& Torr a O 10-3 Torr 300
-/ u
20t
A w
,,- 10¢ •
o
ml
0
100
0
0
I
200
o
I
o
o
o
b
300 400 TIME (Hr$)
I
500
600
Fig. 4. Number of repeated operation cycles of the sensor at 5000C.
compared to pure undoped titanium oxide samples. A comparable result has also been observed in 0.4 wt% Nb doped samples which exhibited a maximum change in resistivity at 450°C. These results show that a lower operating temperature of the sensor can be achieved by doping low Nb concentrations. Also, the maximum change in the resistivity was observed in Nb doped samples. Pure TiO 2 samples show poor sensitivity for oxygen detection because of their low catalytic activity at the optimum operating temperature [8]. The catalytic activities can be increased by the addition of noble metals such as Pd or Pt [9]. The sensitivity can also be increased by the use suitable dopants which increase the electrical conductivity by providing additional charge carriers. As the Nb concentration increases, the resistivity of the samples decreases due to a more enhanced contribution of Nb donor ions. Fig. 4 shows the number of operating cycles carried out on the samples at 500°C for several hours. The material shows no degradation after a number of operating cycles, and is hence found suitable for long term operations. Fig. 5 shows the response time and the recovery time of a pure TiO 2 and of TiO 2 containing 0.2 and 0.4 wt% Nb sensors at 600°C for a fixed oxygen
650
R.K. Sharma et a l . / Applied Surface Science 92 (1996) 647-650
concentration of 1200 ppm. The sensors containing 0.2 and 0.4 wt% Nb show response times of 6.0 and 6.6 s, respectively, whereas pure TiO 2 samples have a response time of 28 s. When the niobium concentration was increased, the response time increased to 36 s. This may be due to the faster movement of the oxygen vacancies TiO 2 samples containing Nb.
4. Conclusions The present study evaluated the effects of Nb doping of titania pellets used for oxygen sensing applications. It established that Nb incorporation prevents the growth of rutile phase crystallites at 1300°C. By increasing the Nb concentration the crystallite size decreases gradually and hence the material loses its sensitivity for oxygen detection. The maximum sensitivity was obtained at 0.2 wt% Nb at an operating temperature of 400°C. The response and recovery times of the Nb incorporated sensor are also shorter than in the pure TiO 2 sensor. Repeated cycles have almost no effect on the sensitivity, hence this sensor is suitable for long time operations.
Acknowledgements The authors acknowledge the financial support given by the CSIR
References [1] Y. Shimizu, Y. Fukuyama, H. Arai and T. Seiyama, Oxygen sensor using pcrvoskite-type oxides: measurements of electrical characteristics, in: Fundamentals and Applications of Chemical Sensors, Eds. D. Schuetzle and R. Hammerle, ACS Syrup. Ser. Vol. 309 (1986) pp. 83-100. [2] P.T. Moselay, Sensors Actuators B 6 (1992) 149. [3] D.S. Howarth and A.L. Micheli, Soc. Automotive Engineers, Inc. 840140 (1985) 1.827-1.833. [4] D. Kohl, Sensors Actuators B 1 (199.1) 158. [5] A. Takami, C-eram. Bull. 67 (1988) 1956. [6] A. Takami, T. Matsuura, T. Sekiya, T. Okawa and Y. Watanabe, Soc. Automotive Engineers, Inc. 850381 (1986) 3.1873.199. [7] M. Kamal Akhtar and S. E. Pratsinis, J. Am. Ceram. Soc. 75 (1992) 3408. [8] H. Arai and Y. Shimizu, in: Fundamentals and Applications of Chemical Sensors, F_As.D. Schuetzle and R. Hammefle, ACS Syrup. Ser. Vol. 309 (1986) pp. 83-98. [9] E.M. Logothetis and WJ. Kaiser, Sensors Actuators Chem. B 4 (1983) 333.