Gas-sensing properties of nanocrystalline TiO2

NanaStructured Materials.Vol. 9. pp. 747-750.1997 Elsevia scienceLtd 0 1997Acta Mdallurgica Inc. Printedin theUSA. All rightsresavd 0965-9773197 $17.00+ .@I

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GAS-SENSING PROPERTIES OF NANOCRYSTALLINE Ti02 Hong-Ming Lin , Chia-Hsi Keng and Chiun-Yen Tung Department of Materials Engineering, Tatung Institute of Technology Taipei, Taiwan, Republic of China Abstract-Nanocrystailine (NC) materials, exhibiting smail particle size and large surface area, may be applied to gas sensors for which an excellent surface eflect is required In this study, NC Ti is synthesized by the gas-condensation method in 10 mbar helium atmosphere and 500 mbar oxygen is backfilled into the chamber to oxidize the NC Ti. NC TiOt are then doped with NC Pr to improve the sensitivity and response time of the sensors NC TiOz and NC IWTiOz sensors are compared by examining the relationship between the operation temperature and sensitivity in CO and NO2 gases. The optimal operation temperature for NO2 and CO gases is about 190 “Cfor the NC TiOt sensor and about 170 “Cfor the NC pt/Ti02 sensor. The maximum sensitivity of about 14 for the NC TiOt sensor is observed with 100 ppm NO, gas and the response time is about l-3 minutes. 01997 Acta Metallurgica Inc. INTRODUCTION Nanocrystalline materials (NCM) with a particle size smaller than 100 nm exhibit many amazing properties which are not found in conventional materials(l-2). One of the distinctive features, the main one for gas sensors, is an extremely large specific surface area. Gas sensors are increasing important and in great demand due to the worsening problem of air pollution and the requirement of industry. The metal oxide semiconductor (MOS) type is the most popular because of its quick response. The traditional MOS sensors are based on thin films or the Schottky diode(3-4). In 1990, a Nb205 doped TiOz-based sensor for sensing humidity was investigated by Katayama(5). At the same time, Hara et a1.(6) developed a semiconductor pH sensor using a NbzOsdoped Ti07 single crystal as a pH-sensitive electrode for use in high-temperature aqueous solutions up to 250°C. In 1993, Takao et al.(7) determined fish freshness which decreases with the increasing concentration of (CH3)Ns Lin et al. (8) synthesized NC TiOz by means of the gas(TMA) by Rudoped Ti4. condensation method as a sensing material and a maximum sensitivity of about 70 was detected in a 250 ppm H$YAir mixture at 500°C. In this study, NC TiOz is used to detect NO, and CO gases. EXPERIMENTAL SiO2 (10x lOmmx0.5mm) is used as a substrate. Silver paste is printed on the Si02 substrate to form sensor electrodes. In this study, a tantalum boat is used as a resistance heating source for evaporation. After NC Ti is produced, pure oxygen is backfilled into the chamber to form NC TiOl. Then, the NC noble metal (Pt) is deposites on the surface of the 747

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H-M LIN, C-H KENGAND C-Y TUNG

NC TiO2under a pressure of 3× 10"5 torr and forms an uniform distribution of dopants. The geometry of the sensor is shown in Figure 1. The sample is placed in a glass chamber and a thermocouple is attached for measuring the operation temperature. The detected CO or NCh gases flow through the chamber continuously in dynamic equilibrium with 500 square cubic centermeter per minute(SCCM) flow rate. The sample is heated by a Ni-Cr ribbon to control the operation temperature. A LCR meter is linked to the computer with an IEEE-488 interface card to record the data directly as shown in Figure 2. SEM is used to observe the morphology and sintering properties of the sensor. The composition is analyzed by SEM/EDS. TEM and HRTEM are used to determine the particle size, crystal structure and the interface structure between the dopant and titanium oxide. Mase Aow n m t ~

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Figure 1 : The geometry of the sensor.

Figure 2 : The measurement system.

RESULTS AND DISCUSSION

Properties of the NC TiOz sensor The TEM image and diffraction pattern indicate a TiO2 mille structure for NC TiO2. The mean particle size measured from TEM images is about 34 nm. NC Pt with about 7 nm diameter is deposited on the surface. Figure 3 shows the secondary electron image of 4.5 wt% Pt doped TiO2. The image indicates the surfaces of the sensor after sintering are porous and uniform. The HRTEM image in Figure 4 shows that good interfacial bonding between the NC Pt and nanophase TiO2 can be achieved.

Sensitivity and Response Times of the TiOz sensor The general definition of the sensitivity is the ratio of the resistance in the pressure of the detected gas to that in air. Figure 5 shows the sensitivity of NC TiO2 and NC 4.5wt%Pt/TiO2 sensors at 0-400 ppm NO2 concentrations. The optimal operation temperatures of NC TiO2 and NC 4.5wt%Pt/TiO2 sensors arc about 190°C and 170°C, respectively. The reason for this is that the work function of Pt-doped NC TiO2 is lower than that of undoped NC TiO2, which enhances the reaction at low temperature. The sensitivity versus NO2 concentrations at the optimal operation temperature of NC TiO2 and NC 4.5wtYdWl"iO2 sensors is shown in Figure 6. The maximum sensitivities of NC TiO2 and NC 4.5wt%PtfTiO2 sensors are about 14 and 10, respectively, at 100 ppm NO2 concentration. These results reveal that 4.5wt%Pt doping in NC TiO2 will not enhance the sensitivity to NO2 but will lower the optimal operation temperature. In 200 ppm or 400 ppm NO2, the sensitivity decreases due to reaching the saturated current density(9). The concentration of CO gas varies in the range of 0-500 ppm. Figure 7 shows the sensitivity of NC TiO2 and NC 4.5wt%Pt/TiO2 sensors at various temperatures and CO

GAs-SENSING PROPERTIES OF NANOCRYSTALUNE "nO 2

749

concentrations. The optimal operation temperatures of these two sensors for detecting CO gas are the same as that for detecting NO2 gas. The results show that the maximum sensitivities of NC TiO2 and NC 4.Swt%Pt/TiO2 sensors are about 10 and 8, respectively, at 100 ppm CO gas. Figure 8 shows the effect of CO concentration on the sensitivity of NC TiO2 and NC 4.Swt%Pt/TiO2 sensors. In 200 ppm or 500 ppm CO, the sensitivity also decreases due to reaching of its saturated current density as mention in the previous section.

Figure 3 : The secondary electron image of NC 4.5wt% Pt/TiO2 after sintering.

Figure 4 • HRTEM image of NC Pt-doped NC TiO2.

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H-M LIN, C-H KENGANDC-Y TUNG

The response time is influenced by the operation temperature, the type of dopant, the dopant concentrations, and the concentration of the detected gas. An IBM PC is used to record the response time at different gas concentrations. The test chamber is maintained in dynamic equilibrium at various concentrations and the response time is determined at the moment that the resistance is stable. The response times of NC TiO2 and NC 4.5wt% It/ TiO2 sensors in detecting NO2 and CO are about 1~3 minutes. CONCLUSIONS NanocrystaUine TiO2 and 4.5wt%Pt/TiO2 sensors are used to measure the diifercnt NO2 and CO concentrations. The optimal operation temperature of NC 4.Swt%Pt/TiO2 sensor prepared in a 10 mbar helium atmosphere is lower than that of a NC TiO2 sensor. This indicates that a Pt dopant can reduced the operation temperature. The optimal operation temperature of NC TiO2, NC 4.Swt%Pt/ TiO2 sensors prepared in a 10 mbar helium atmosphere in detecting NO: or CO gas, are 190°C and 170°C, respectively. Response is linear at 0-100 ppm for NC TiO2, NC 4.SwP/d't/TiO2 sensors for detecting NO: or CO gas. When the concentration is greater than 100 ppm, the sensitivity will decrease because it reaches the saturated current density. The response time is influenced by the operation temperature, the type of dopant, the dopant concentration and the concentration of the detected gases. In 0~100 ppm CO gas, the sensitivity has a more linear characteristic for NC TiO2 sensors. The response time of NC TiO2 and NC Pt/TiO2 sensors for detecting NO2 and CO are both about 1~3 minutes. ACKNOWLEDGMENTS We would like to thank the National Science Council, Republic of China for financial support through Contract Number NSC 84-2216-E-036-021. REFERENCES 1. X.Zhu, R. Birringer, U. Herr and H. Gleiter, Phys. Rev. B, 35, 9085(1987). 2. R.W. Siegal and H. Hahn, in Current Trends in the Physics of Materials, edited by M. Yussouff, (1987), 403. 3. N. Yamamoto, Y. Fujita, O. Ando, and H. Tsubomura, Surface Science, 146, 10(1984). 4. S.J. Fonash, Zheng Li, and M. J. O'Leary, J. Appi. Phys~ 58 (11), 4415(1985). 5. K. Katayama, K. Hasegawa, Y. Takahashi, and T. Akiba, Sensors and Actuators A, 24, 55(1990). 6. Nobuyoshi Hara, and Katsuhisa Sugimoto, J. Electrochem. Sock, 137 (8), 2517(1990). 7. Y. Takao, Y. Iwanaga, Y. Shimizu, and M. Egashira, Sensors and Actuators B, 10, 229 and 235(1993). 8. H.M. Lin, T. Y. I-Isu, C. Y. Tung, C. M. Hsu, NanostructuredMaterials, 6, 1001(1995). 9. A. Mandelis, C. Christofides, Phys., Chem. and Tech. of Solid State Gas Sensor Devices, John Wiley& Sons, Inc., New York, 1993, p. 14.