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The preparation and oxygen-sensing properties of α-Ti3O5 thin film
Sensors and Actuators B 88 (2003) 115–119
The preparation and oxygen-sensing properties of a-Ti3O5 thin film Liaoying Zheng* The State Key Lab of High Performance Ceramics and Superfinemicrostructure, Shanghai Institute of Ceramics, Chinese Academy of Science, Shanghai 200050, China Received 14 March 2002; received in revised form 10 August 2002; accepted 15 August 2002
Abstract In this paper, the preparation and oxygen-sensing properties of a-Ti3O5 film are studied. By reducing TiO2 thin films in H2 atmosphere at 1200 and 1250 8C, the Ti3O5 film with orthorhombic structure (a phase) can be obtained. The a-Ti3O5 film is stable up to 650 8C in air. The results of property investigation show that the a-Ti3O5 film has low resistance-temperature coefficient and certain oxygen-sensing properties, however, its oxygen-sensing properties need further improvement. To explain the oxygen-sensing mechanism of the a-Ti3O5 film, the surface phase transition hypothesis is raised. # 2002 Elsevier Science B.V. All rights reserved. Keywords: Oxygen-sensing; a-Ti3O5; Film mechanism
oxide (TiOx, 1 < x < 2) could be obtained at different oxygen partial pressure.
1. Introduction As the continuously rising demand for the protection of atmosphere, the sensor controlling air fuel ratio (A/F) is needed widely [1,2]. The sensing materials now used are mainly TiO2 and ZrO2-based bulk materials. But their output is temperature-dependent and then complex compensating circuit is needed [3,4]. To overcome the shortage, a new material—a-Ti3O5 is researched. a-Ti3O5 is a metal-like compound and has high concentration of quasi-free electron, thus, its resistancetemperature coefficient is very small. It is a non-stoichiometric semiconducting oxide and its atom ratio of oxygen and titanium can be changed from 1.66 to 1.70 in different oxygen partial pressure [5]. Thus, the a-Ti3O5 material is a potential material for oxygen sensor. Several synthesis methods can be used for titanium oxide (TiOx, 1 < x < 2) [6–9]: (1) Reduction process: reduction of TiO2 in H2, CO atmosphere or by C, Ti under vacuum or Ar atmosphere. (2) Physical vapor deposition (PVD) or chemical vapor deposition (CVD): using Ti metal, inorganic or organic salt of titanium as raw material, the different titanium * Fax: þ86-21-5241-3122. E-mail address: [email protected] (L. Zheng).
In this paper, the preparation and oxygen-sensing properties of a-Ti3O5 thin films are investigated.
2. Experimental First, TiO2 thin films were prepared on Al2O3 substrates by sol–gel method: using Ti(OC4H9)4 as raw material, absolute ethanol as a solvent and HCl. Acetyl acetone as stabilizing agent, then a high stable sol with 0.8 mol l1 Ti4þ ion was achieved. All of the chemicals are reagent grade. Then TiO2 thin films were formed by heating at 600 8C for 30 min after dip-coating. The certain thickness such as 2 mm could be reached by repeating dip-coating and heating process. Such TiO2 thin films were reduced in H2 atmosphere at different temperature to synthesize Ti3O5 thin films. The X-ray diffraction (XRD, Cu Ka 40 kV, 40 mA) was used to determine the phase structure of the films. The morphology of the films was observed by scanning electron microscope (SEM, XL30ESEM). X-ray photoelectron spectrum (XPS) was also used to determine the valence state of the Al ion on the surface of thin films. The relative properties of thin films were tested in our lab, and the set-up was shown in Fig. 1.
0925-4005/02/$ – see front matter # 2002 Elsevier Science B.V. All rights reserved. PII: S 0 9 2 5 - 4 0 0 5 ( 0 2 ) 0 0 3 0 2 - 7
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L. Zheng / Sensors and Actuators B 88 (2003) 115–119
Fig. 1. Apparatus for testing oxygen-sensitivity of the film element.
3. Results and discussion 3.1. The preparation of Ti3O5 thin film The XRD was used to measure the phase structure in the films reduced at different temperature for 4 h in H2 atmosphere (Fig. 2). From the XRD spectrum, the single Ti3O5 structure could be formed when the temperature is above 1200 8C. No rutile phase is remnant. Then the 1200 8C is determined to synthesize the Ti3O5 films in this paper. The phase of Ti3O5 is a-Ti3O5 with orthorhombic structure (or pseudobrookite structure). It is reported that the a-Ti3O5 has a phase transition to bTi3O5 (with monoclinic structure) at 160 8C [10]. However, in our experiment, the a-Ti3O5 phase is kept to room temperature, and the phase transition process does not occur. XPS analysis (Fig. 3) digs out that some Al ions in Ti3O5 thin films and the chemical shift of Al3þ is about 0.6 eV.
Fig. 3. The binding energy of Al in Ti3O5 film.
Therefore, the deformation equation of Al ion substituted to Ti sites would be: Ti3 O5
1=30
Ti3 O5
1=30
3Al2 O3 ! 6AlTi þ Vo þ 9Oo 5Al2 O3 ! 9AlTi þ Ali þ 15Oo Because of small radius of Al ions and large octahedron space in a-Ti3O5 structure, both Al interstitial ions and oxygen vacancies could be formed. According to Sperisen and Mocellin [10], the lattice parameters of a-Ti3O5 are very sensitive to the impurity level. For small trivalent ions (i.e. Al3þ in this research) can easily replace Ti sites and consequently decrease the distortion of the [TiO6] octahedron, then the phase transition temperature can be decreased under room temperature. 3.2. The morphology of a-Ti3O5 thin films The SEM image of a-Ti3O5 thin films is shown in Fig. 4. The average grain size of the film is about 1.0 mm and there are some pores distributed among the grains. Comparing to
Fig. 2. The XRD result of the TiO2 film reduced by H2 at different temperature for 4 h.
Fig. 4. The SEM image of the film (a) a-Ti3O5 film; (b) TiO2 film (1000 8C).
L. Zheng / Sensors and Actuators B 88 (2003) 115–119
Fig. 5. The XRD result of the a-Ti3O5 film after heating at different temperature in air.
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characters of the film are advantage to the design of electric circuit. From the relationship of resistivity and temperature, the high concentration of quasi-free electrons in conduction band and low activity energy for electron conduction of the a-Ti3O5 thin films could be verified, which is just like metal. Nevertheless, because the conduction band is very narrow in a-Ti3O5 structure [11], the electrons in conduction band are high correlated. Then the movement of electrons needs to overcome extra resistance of potential field, so the films’ resistance is higher than metal’s in room temperature and its resistivity decreased with the increase of temperature, whereas the resistivity always increase with the temperature in metal. 3.4. Oxygen-sensing properties
the TiO2 thin films annealed at 1000 8C, a small quantity of pores and large mean size of grains in the a-Ti3O5 thin films could be found, which was beneficial to long-term stability of the films but unfavorable to oxygen-sensing properties. 3.3. The structure stability and resistivity-temperature character of the a-Ti3O5 thin films The structure stability at high temperature of the a-Ti3O5 thin films was discovered by XRD after heating them at different temperature in air (Fig. 5). The result shows that no structure transition occurs. Up to 700 8C, the films are oxidized to TiO2 (rutile structure). Therefore, the oxygensensing properties of the a-Ti3O5 thin films were measured under 650 8C. The resistivity-temperature character of the a-Ti3O5 thin films was measured in N2 atmosphere (Fig. 6). The films exhibit low resistivity-temperature coefficient in N2 atmosphere. From 200 to 600 8C, the resistivity changed small. The coefficient of resistance-temperature is about 2:0 103 K1, whereas the resistance-temperature coefficient of TiO2 film is greater than 101 K1. Additionally, comparing to that of TiO2 thin film (>100 MO), the film’s resistance at room temperature is also small (about 10–30 kO). These
Fig. 6. The change of resistance of a-Ti3O5 and TiO2 film with the temperature.
The oxygen sensitivity of the a-Ti3O5 thin films was interrogated in H2/N2 and O2/N2 atmosphere (Fig. 7, the sign () in abscissa only represents the reduction atmosphere in this paper). The resistance of the Ti3O5 thin films has a sudden change from 16.2 to 56.2 kO as the atmosphere is changed from reduction to oxidation at 600 8C. The oxygen sensitivity in 400 and 500 8C are also shown to verify the good temperature stability of the films. The response time of the a-Ti3O5 thin films is shown in Fig. 8,
Fig. 7. The oxygen sensitivity of a-Ti3O5 film.
Fig. 8. The response time to the atmosphere of a-Ti3O5 film.
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they disabsorbed from the surface. The decrease of oxygen ion of the surface leads to a0 –a phase transition, and the resistance of the film is decreased. Using this hypothesis, the oxygen-sensing properties of aTi3O5 can be explained easily. The demonstration of the hypothesis will be published in other paper.
4. Conclusion
Fig. 9. The reproducibility of the a-Ti3O5 film.
where tH2 !O2 is the response time when the atmosphere is changed from reduction (1 vol.% H2 in N2) to oxidation (1 vol.% O2 in N2), and tO2 !H2 is the response time when the atmosphere is changed from oxidation to reduction. The response time of a-Ti3O5 is about 11.2 and 11.8 s to reduction and oxidation atmosphere, respectively. The reproducibility of the a-Ti3O5 thin films is illustrated with the curve of logarithm resistance and time while the circumstance is changed repeatedly (Fig. 9). The 1 vol.% H2 and 1 vol.% O2 in N2 atmosphere are represented to reduction and oxide atmosphere, respectively. It can be seen from Fig. 9 that the films have good reproducibility. 3.5. Oxygen-sensing mechanism of a-Ti3O5 films To explain the mechanism of oxygen-sensing materials in the type of resistance-changing, some models such as interface potential barrier model, oxygenic ion trap potential barrier model, etc. have been raised [12]. But these models are developed from the wide forbidden band materials with low concentration electrons. However, the a-Ti3O5 has its peculiar energy band structure, like metal, so these models are unsuitable to this material. Thus, a new mechanism—surface phase-transition hypothesis is raised. The description of the hypothesis is as follows: In oxidation atmosphere, the oxygen molecules can be physicals-absorbed in the surface of the film, then they dissociated into atoms and ionized. Driven by the concentration difference between the surface and inner of the film, the ionized oxygen ions can enter the lattice of the film, which causes the surface a–a0 phase transition. Because the a0 phase has the semiconductor-type energy band and low conduction ability, the films’ resistance is increased. On the contrary, in reduction atmosphere, the oxygen ions on the surface react with the reduction atom (such as H atom), then
(a) The a-Ti3O5 film can be obtained by reduced TiO2 thin films in H2 atmosphere at 1200 and 1250 8C. (b) When cooled a-Ti3O5 film to room temperature, no phase transition to b-Ti3O5 is founded, which is caused probably by the existence of Al ions. (c) The a-Ti3O5 film is stable in air when the temperature is lower than 650 8C. (d) The a-Ti3O5 film has good temperature stability in resistance and oxygen sensitivity. (e) The a-Ti3O5 film has certain oxygen-sensing properties, but its oxygen-sensing properties need further improvement. (f) The surface phase transition hypothesis is raised to explain the mechanism of the oxygen-sensing properties of a-Ti3O5 films.
Acknowledgements The author gratefully acknowledges the support of National Natural Science Foundation of China.
References [1] N. Yamazoe, N. Miura, Development of gas sensors for environmental protection, IEEE Trans. Compon. Packag. Manuf. Technol., Part A 18 (1995) 252–256. [2] S. Hasegawa, Y. Sasaki, S. Matsuhara, Oxygen-sensing factor of TiO2 doped with metal ions, Sens. Actuators, B, Chem. 13 (1993) 509–510. [3] L.Y. Zheng, M.X. Xu, T.X. Xu, TiO2x thin films as oxygen sensor, Sens. Actuators, B, Chem. 66 (2000) 28–30. [4] A.L. Micheli, Fabrication and performance evaluation of a titania automotive exhaust gas sensor, Ceram. Bull. 63 (5) (1984) 694– 698. [5] I.E. Grey, C. Li, I.C. Madsen, Phase equilibria and structural studies on the solid solution, J. Solid State Chem. 113 (1994) 62–73. [6] J.F. Baumard, D. Panis, A.M. Anthony, A study of Ti–O system between Ti3O5 and TiO2 at high temperature by means of electrical resistivity, J. Solid State Chem. 20 (1977) 43–51. [7] A. Afir, M. Achour, A. Pialoux, Etude de la re´ duction carbothermique du dioxyde de titane par diffraction X a` haute tempe´ rature sous pression controˆ le´ e, J. Alloys Compounds 210 (1994) 201– 208. [8] A. Afir, M. Achour, N. Saoula, X-ray diffraction study of Ti–O–C system at high temperature and in a continuous vacuum, J. Alloys Compounds 288 (1999) 124–140. [9] G.A. Swift, R. Roc, Formation studies of TiC from carbon coated TiO2, J. Mater. Sci. 34 (1999) 3083–3093.
L. Zheng / Sensors and Actuators B 88 (2003) 115–119 [10] T. Sperisen, A. Mocellin, On structures of mixed titanium aluminum oxides, J. Mater. Sci. Lett. 10 (1991) 831–833. [11] T. Kitamura, K. Shibata, K. Takeda, In-flight reduction of Fe2O3, Cr2O3, TiO2 and Al2O3 by Ar–H2 and Ar–CH4 plasma, ISI Int. 33 (11) (1993) 1150–1158. [12] T.X. Xu, Electronic Ceramics, Tianjin University, Tianjin, China, 1st ed., 1990, p. 315.
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Biography Liaoying Zheng was born in 1974 in Zhejiang of China. He obtained his doctor degree in Tianjin University. Now he is engaged in his postdoctoral work on piezoelectric materials and inorganic film in Laboratory of Functional Inorganic Materials, Shanghai Institute of Ceramics, Chinese Academy of Science.
The preparation and oxygen-sensing properties of a-Ti3O5 thin film Liaoying Zheng* The State Key Lab of High Performance Ceramics and Superfinemicrostructure, Shanghai Institute of Ceramics, Chinese Academy of Science, Shanghai 200050, China Received 14 March 2002; received in revised form 10 August 2002; accepted 15 August 2002
Abstract In this paper, the preparation and oxygen-sensing properties of a-Ti3O5 film are studied. By reducing TiO2 thin films in H2 atmosphere at 1200 and 1250 8C, the Ti3O5 film with orthorhombic structure (a phase) can be obtained. The a-Ti3O5 film is stable up to 650 8C in air. The results of property investigation show that the a-Ti3O5 film has low resistance-temperature coefficient and certain oxygen-sensing properties, however, its oxygen-sensing properties need further improvement. To explain the oxygen-sensing mechanism of the a-Ti3O5 film, the surface phase transition hypothesis is raised. # 2002 Elsevier Science B.V. All rights reserved. Keywords: Oxygen-sensing; a-Ti3O5; Film mechanism
oxide (TiOx, 1 < x < 2) could be obtained at different oxygen partial pressure.
1. Introduction As the continuously rising demand for the protection of atmosphere, the sensor controlling air fuel ratio (A/F) is needed widely [1,2]. The sensing materials now used are mainly TiO2 and ZrO2-based bulk materials. But their output is temperature-dependent and then complex compensating circuit is needed [3,4]. To overcome the shortage, a new material—a-Ti3O5 is researched. a-Ti3O5 is a metal-like compound and has high concentration of quasi-free electron, thus, its resistancetemperature coefficient is very small. It is a non-stoichiometric semiconducting oxide and its atom ratio of oxygen and titanium can be changed from 1.66 to 1.70 in different oxygen partial pressure [5]. Thus, the a-Ti3O5 material is a potential material for oxygen sensor. Several synthesis methods can be used for titanium oxide (TiOx, 1 < x < 2) [6–9]: (1) Reduction process: reduction of TiO2 in H2, CO atmosphere or by C, Ti under vacuum or Ar atmosphere. (2) Physical vapor deposition (PVD) or chemical vapor deposition (CVD): using Ti metal, inorganic or organic salt of titanium as raw material, the different titanium * Fax: þ86-21-5241-3122. E-mail address: [email protected] (L. Zheng).
In this paper, the preparation and oxygen-sensing properties of a-Ti3O5 thin films are investigated.
2. Experimental First, TiO2 thin films were prepared on Al2O3 substrates by sol–gel method: using Ti(OC4H9)4 as raw material, absolute ethanol as a solvent and HCl. Acetyl acetone as stabilizing agent, then a high stable sol with 0.8 mol l1 Ti4þ ion was achieved. All of the chemicals are reagent grade. Then TiO2 thin films were formed by heating at 600 8C for 30 min after dip-coating. The certain thickness such as 2 mm could be reached by repeating dip-coating and heating process. Such TiO2 thin films were reduced in H2 atmosphere at different temperature to synthesize Ti3O5 thin films. The X-ray diffraction (XRD, Cu Ka 40 kV, 40 mA) was used to determine the phase structure of the films. The morphology of the films was observed by scanning electron microscope (SEM, XL30ESEM). X-ray photoelectron spectrum (XPS) was also used to determine the valence state of the Al ion on the surface of thin films. The relative properties of thin films were tested in our lab, and the set-up was shown in Fig. 1.
0925-4005/02/$ – see front matter # 2002 Elsevier Science B.V. All rights reserved. PII: S 0 9 2 5 - 4 0 0 5 ( 0 2 ) 0 0 3 0 2 - 7
116
L. Zheng / Sensors and Actuators B 88 (2003) 115–119
Fig. 1. Apparatus for testing oxygen-sensitivity of the film element.
3. Results and discussion 3.1. The preparation of Ti3O5 thin film The XRD was used to measure the phase structure in the films reduced at different temperature for 4 h in H2 atmosphere (Fig. 2). From the XRD spectrum, the single Ti3O5 structure could be formed when the temperature is above 1200 8C. No rutile phase is remnant. Then the 1200 8C is determined to synthesize the Ti3O5 films in this paper. The phase of Ti3O5 is a-Ti3O5 with orthorhombic structure (or pseudobrookite structure). It is reported that the a-Ti3O5 has a phase transition to bTi3O5 (with monoclinic structure) at 160 8C [10]. However, in our experiment, the a-Ti3O5 phase is kept to room temperature, and the phase transition process does not occur. XPS analysis (Fig. 3) digs out that some Al ions in Ti3O5 thin films and the chemical shift of Al3þ is about 0.6 eV.
Fig. 3. The binding energy of Al in Ti3O5 film.
Therefore, the deformation equation of Al ion substituted to Ti sites would be: Ti3 O5
1=30
Ti3 O5
1=30
3Al2 O3 ! 6AlTi þ Vo þ 9Oo 5Al2 O3 ! 9AlTi þ Ali þ 15Oo Because of small radius of Al ions and large octahedron space in a-Ti3O5 structure, both Al interstitial ions and oxygen vacancies could be formed. According to Sperisen and Mocellin [10], the lattice parameters of a-Ti3O5 are very sensitive to the impurity level. For small trivalent ions (i.e. Al3þ in this research) can easily replace Ti sites and consequently decrease the distortion of the [TiO6] octahedron, then the phase transition temperature can be decreased under room temperature. 3.2. The morphology of a-Ti3O5 thin films The SEM image of a-Ti3O5 thin films is shown in Fig. 4. The average grain size of the film is about 1.0 mm and there are some pores distributed among the grains. Comparing to
Fig. 2. The XRD result of the TiO2 film reduced by H2 at different temperature for 4 h.
Fig. 4. The SEM image of the film (a) a-Ti3O5 film; (b) TiO2 film (1000 8C).
L. Zheng / Sensors and Actuators B 88 (2003) 115–119
Fig. 5. The XRD result of the a-Ti3O5 film after heating at different temperature in air.
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characters of the film are advantage to the design of electric circuit. From the relationship of resistivity and temperature, the high concentration of quasi-free electrons in conduction band and low activity energy for electron conduction of the a-Ti3O5 thin films could be verified, which is just like metal. Nevertheless, because the conduction band is very narrow in a-Ti3O5 structure [11], the electrons in conduction band are high correlated. Then the movement of electrons needs to overcome extra resistance of potential field, so the films’ resistance is higher than metal’s in room temperature and its resistivity decreased with the increase of temperature, whereas the resistivity always increase with the temperature in metal. 3.4. Oxygen-sensing properties
the TiO2 thin films annealed at 1000 8C, a small quantity of pores and large mean size of grains in the a-Ti3O5 thin films could be found, which was beneficial to long-term stability of the films but unfavorable to oxygen-sensing properties. 3.3. The structure stability and resistivity-temperature character of the a-Ti3O5 thin films The structure stability at high temperature of the a-Ti3O5 thin films was discovered by XRD after heating them at different temperature in air (Fig. 5). The result shows that no structure transition occurs. Up to 700 8C, the films are oxidized to TiO2 (rutile structure). Therefore, the oxygensensing properties of the a-Ti3O5 thin films were measured under 650 8C. The resistivity-temperature character of the a-Ti3O5 thin films was measured in N2 atmosphere (Fig. 6). The films exhibit low resistivity-temperature coefficient in N2 atmosphere. From 200 to 600 8C, the resistivity changed small. The coefficient of resistance-temperature is about 2:0 103 K1, whereas the resistance-temperature coefficient of TiO2 film is greater than 101 K1. Additionally, comparing to that of TiO2 thin film (>100 MO), the film’s resistance at room temperature is also small (about 10–30 kO). These
Fig. 6. The change of resistance of a-Ti3O5 and TiO2 film with the temperature.
The oxygen sensitivity of the a-Ti3O5 thin films was interrogated in H2/N2 and O2/N2 atmosphere (Fig. 7, the sign () in abscissa only represents the reduction atmosphere in this paper). The resistance of the Ti3O5 thin films has a sudden change from 16.2 to 56.2 kO as the atmosphere is changed from reduction to oxidation at 600 8C. The oxygen sensitivity in 400 and 500 8C are also shown to verify the good temperature stability of the films. The response time of the a-Ti3O5 thin films is shown in Fig. 8,
Fig. 7. The oxygen sensitivity of a-Ti3O5 film.
Fig. 8. The response time to the atmosphere of a-Ti3O5 film.
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L. Zheng / Sensors and Actuators B 88 (2003) 115–119
they disabsorbed from the surface. The decrease of oxygen ion of the surface leads to a0 –a phase transition, and the resistance of the film is decreased. Using this hypothesis, the oxygen-sensing properties of aTi3O5 can be explained easily. The demonstration of the hypothesis will be published in other paper.
4. Conclusion
Fig. 9. The reproducibility of the a-Ti3O5 film.
where tH2 !O2 is the response time when the atmosphere is changed from reduction (1 vol.% H2 in N2) to oxidation (1 vol.% O2 in N2), and tO2 !H2 is the response time when the atmosphere is changed from oxidation to reduction. The response time of a-Ti3O5 is about 11.2 and 11.8 s to reduction and oxidation atmosphere, respectively. The reproducibility of the a-Ti3O5 thin films is illustrated with the curve of logarithm resistance and time while the circumstance is changed repeatedly (Fig. 9). The 1 vol.% H2 and 1 vol.% O2 in N2 atmosphere are represented to reduction and oxide atmosphere, respectively. It can be seen from Fig. 9 that the films have good reproducibility. 3.5. Oxygen-sensing mechanism of a-Ti3O5 films To explain the mechanism of oxygen-sensing materials in the type of resistance-changing, some models such as interface potential barrier model, oxygenic ion trap potential barrier model, etc. have been raised [12]. But these models are developed from the wide forbidden band materials with low concentration electrons. However, the a-Ti3O5 has its peculiar energy band structure, like metal, so these models are unsuitable to this material. Thus, a new mechanism—surface phase-transition hypothesis is raised. The description of the hypothesis is as follows: In oxidation atmosphere, the oxygen molecules can be physicals-absorbed in the surface of the film, then they dissociated into atoms and ionized. Driven by the concentration difference between the surface and inner of the film, the ionized oxygen ions can enter the lattice of the film, which causes the surface a–a0 phase transition. Because the a0 phase has the semiconductor-type energy band and low conduction ability, the films’ resistance is increased. On the contrary, in reduction atmosphere, the oxygen ions on the surface react with the reduction atom (such as H atom), then
(a) The a-Ti3O5 film can be obtained by reduced TiO2 thin films in H2 atmosphere at 1200 and 1250 8C. (b) When cooled a-Ti3O5 film to room temperature, no phase transition to b-Ti3O5 is founded, which is caused probably by the existence of Al ions. (c) The a-Ti3O5 film is stable in air when the temperature is lower than 650 8C. (d) The a-Ti3O5 film has good temperature stability in resistance and oxygen sensitivity. (e) The a-Ti3O5 film has certain oxygen-sensing properties, but its oxygen-sensing properties need further improvement. (f) The surface phase transition hypothesis is raised to explain the mechanism of the oxygen-sensing properties of a-Ti3O5 films.
Acknowledgements The author gratefully acknowledges the support of National Natural Science Foundation of China.
References [1] N. Yamazoe, N. Miura, Development of gas sensors for environmental protection, IEEE Trans. Compon. Packag. Manuf. Technol., Part A 18 (1995) 252–256. [2] S. Hasegawa, Y. Sasaki, S. Matsuhara, Oxygen-sensing factor of TiO2 doped with metal ions, Sens. Actuators, B, Chem. 13 (1993) 509–510. [3] L.Y. Zheng, M.X. Xu, T.X. Xu, TiO2x thin films as oxygen sensor, Sens. Actuators, B, Chem. 66 (2000) 28–30. [4] A.L. Micheli, Fabrication and performance evaluation of a titania automotive exhaust gas sensor, Ceram. Bull. 63 (5) (1984) 694– 698. [5] I.E. Grey, C. Li, I.C. Madsen, Phase equilibria and structural studies on the solid solution, J. Solid State Chem. 113 (1994) 62–73. [6] J.F. Baumard, D. Panis, A.M. Anthony, A study of Ti–O system between Ti3O5 and TiO2 at high temperature by means of electrical resistivity, J. Solid State Chem. 20 (1977) 43–51. [7] A. Afir, M. Achour, A. Pialoux, Etude de la re´ duction carbothermique du dioxyde de titane par diffraction X a` haute tempe´ rature sous pression controˆ le´ e, J. Alloys Compounds 210 (1994) 201– 208. [8] A. Afir, M. Achour, N. Saoula, X-ray diffraction study of Ti–O–C system at high temperature and in a continuous vacuum, J. Alloys Compounds 288 (1999) 124–140. [9] G.A. Swift, R. Roc, Formation studies of TiC from carbon coated TiO2, J. Mater. Sci. 34 (1999) 3083–3093.
L. Zheng / Sensors and Actuators B 88 (2003) 115–119 [10] T. Sperisen, A. Mocellin, On structures of mixed titanium aluminum oxides, J. Mater. Sci. Lett. 10 (1991) 831–833. [11] T. Kitamura, K. Shibata, K. Takeda, In-flight reduction of Fe2O3, Cr2O3, TiO2 and Al2O3 by Ar–H2 and Ar–CH4 plasma, ISI Int. 33 (11) (1993) 1150–1158. [12] T.X. Xu, Electronic Ceramics, Tianjin University, Tianjin, China, 1st ed., 1990, p. 315.
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Biography Liaoying Zheng was born in 1974 in Zhejiang of China. He obtained his doctor degree in Tianjin University. Now he is engaged in his postdoctoral work on piezoelectric materials and inorganic film in Laboratory of Functional Inorganic Materials, Shanghai Institute of Ceramics, Chinese Academy of Science.