Gas sensing properties of WO 3 doped rutile TiO 2 thick film at high operating temperature Sung-Eun Jo, Byeong-Geun Kang, Sungmoo Heo, Soonho Song, Yong-Jun Kim * School of Mechanical Engineering, Yonsei University, Seoul 120-749, Republic of Korea article info Article history: Received 31 March 2009 Received in revised form 29 June 2009 Accepted 29 June 2009 Available online xxxx PACS: 51.50.+v Keywords: WO 3 doped rutile TiO 2 High temperature heat treatment Grain growth High temperature gas sensor abstract A semiconductor gas sensor based on WO 3 doped TiO 2 having a rutile phase was fabricated on an Al 2 O 3 substrate. The sensing film of the sensor was deposited by using screen printing. In order to enhance the sensitivity of the sensor, the sensing film was fabricated with a porous shape by high temperature heat treatment and the TiO 2 layer was doped with WO 3 to improve the gas selectivity. The surface topography and inner morphological properties of the sensing film were characterized with scanning electron microscopy (SEM), atomic force microscopy (AFM) and X-ray diffraction (XRD). The gas sensing proper- ties of the fabricated sensor were evaluated by detecting NO 2 and other oxidizing gases (CO, O 2 and CO 2 ) at high operating temperature (600 ° C). Ó 2009 Elsevier B.V. All rights reserved. 1. Introduction Semiconductor gas sensors can detect oxidizing and reducing gases by measuring the conductance changes in the surface phe- nomena in terms of gas adsorption and desorption [1,2]. TiO 2 is a well known metal oxide semiconductor. It can be used for high temperature gas sensors which are more suitable for controlling many high temperature technological processes such as the en- gine’s work of automobiles. TiO 2 exists in several crystalline mod- ifications, the common forms being anatase and rutile [3]. Anatase is transferred to rutile at above 600 °C [4], so that anatase TiO 2 based sensors have relatively low heat treatment temperature lev- els [5,6]. However, low heat treatment temperature can induce grain growth of the sensing films in sensors operating at high temperature ($500 °C) and decreases the sensors’ sensitivity [7]. It is known a specific surface area of the sensing film is decreased with increase of grain size [1]. So, the grain growth of the sensing film induces low sensitivity of the sensor. On the other hand, ther- mally stable, rutile TiO 2 based gas sensors can tolerate high heat treatment temperature levels. Therefore, rutile TiO 2 based gas sensors can detect gases at high operating temperature with no grain growth. Although, the high heat treatment temperature ensures stable operation of the sensors at high temperature, it causes grain growth of the sensing films during the heat treat- ment process. Therefore, in this study, WO 3 doped rutile TiO 2 was used as the material of the sensing film to reduce the grain growth of the film during the heat treatment process. The sensing film was deposited on an Al 2 O 3 substrate by screen printing and was thermally treated at high temperature (1100 °C). The surface and morphological properties of the fabricated sensor were studied, while its gas sensing properties were investigated by detecting NO 2 and other oxidizing gases (CO, O 2 and CO 2 ) at high operating temperature. 2. Experimental The sensor was fabricated on an alumina substrate. The sensing film was deposited by screen printing on the IDT (interdigitated) electrode. A printable paste for screen printing was prepared by using an organic vehicle based on a terpineol. The paste consisted of 16 wt.% nanopowders (average particle size of TiO 2 :20nm, WO 3 : 30 nm) and 84 wt.% solvent ( a terpineol, ethyl cellulose, dis- persing agent). The 1.5 l m thick sensing film was heat treated at 1100 °C for 1.5 h in dry air condition to define a porous shaped sensing film and enhance the thermal stability of the sensor. The surface topography of the screen printed sensing film was investigated with an S4800 scanning electron microscope (SEM) operated at 15 keV and an XE150 atomic force microscope (AFM). The structural analysis was carried out by using a D/MAX2500H X-ray diffractometer (XRD). 1567-1739/$ - see front matter Ó 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.cap.2009.06.053 * Corresponding author. Tel.: +82 2 2123 2844. E-mail address: yjk@yonsei.ac.kr (Y J. Kim). Current Applied Physics xxx (2009) xxx–xxx Contents lists available at ScienceDirect Current Applied Physics journal homepage: www.elsevier.com/locate/cap ARTICLE IN PRESS Please cite this article in press as: S E. Jo et al., Gas sensing properties of WO 3 doped rutile TiO 2 thick film at high operating temperature, Curr. Appl. Phys. (2009), doi:10.1016/j.cap.2009.0 6.053 Fig. 1. SEM images of the sensing films. Fig. 2. AFM images of the sensing films. Fig. 3. XRD patterns of undoped TiO 2 , 11 wt.% WO 3 doped TiO 2 and 15 wt.% WO 3 doped TiO 2 . 2 S E. Jo et al. / Current Applied Physics xxx (2009) xxx–xxx ARTICLE IN PRESS Please cite this article in press as: S E. Jo et al., Gas sensing properties of WO 3 doped rutile Ti O 2 thick film at high operating tempe rature, Curr. Appl. Phys. (2009), doi:10.1016/j.cap.2009.06.053 To characterize the fabricated sensor, it was used to detect NO 2 and other oxidizing gases (CO, CO 2 and O 2 ). The sensor was oper- ated at 600 °C. The ratio of the WO 3 doping into TiO 2 was varied among 0 (undoped), 11 and 15 wt.%. 3. Results and discussion Fig. 1 shows SEM images of the sensing films deposited by screen printing. The sensing film exhibited the typical microstruc- ture of screen printed sensing films. A porous sensing film surface was obtained due to the high heat treatment temperature. The WO 3 doping did not induce any change. AFM micrographs of the undoped, and 11 wt.% and 15 wt.% WO 3 doped TiO 2 films are shown in Fig. 2. The AFM micrographs con- firmed that the WO 3 doping was not a dominant factor in defining the porous and rough surface of the sensing films. The crystal structure of the sensing films was revealed in the XRD patterns shown in Figs. 3 and 4. Although, the WO 3 doping ef- fect on the surface topography was not clearly evident, the inner morphological properties of the sensing film were affected by the WO 3 doping. Fig. 3 revealed that the sensing films consisted of al- most a single phase of rutile TiO 2 , while Fig. 4 revealed the (1 1 0) peak of the rutile TiO 2 of each film. The peaks became broader and lower with increasing amount of doped WO 3 . The grain sizes of the sensing films were calculated by the Scherrer equation (1) t ¼ 0:9 k=B cos h B : ð1Þ In this study, we used TiO 2 nanopowder with a particle size of 20 nm. After heat treatment, the grain sizes of the undoped, and 11 wt.% and 15 wt.% WO 3 doped TiO 2 films were 102.2 nm, 68.1 nm and 62.9 nm, respectively. Following the heat treatment, the grain sizes of the fabricated sensing films increased. But, WO 3 doping disturbed the grain growth of the sensing films. That is, the WO 3 doping clearly affected the grain growth of the TiO 2 particles. Fig. 5 shows the results of NO 2 (500 ppm) detection at 600 °C. The undoped and two WO 3 doped TiO 2 sensors exhibited a gas sensing property similar to that of a p-type, semiconductor gas sensor. The sensitivity to gases is defined as (R a À R g )/R g , where R a and R g are the steady state resistances of the device in null (N 2 ) and oxidizing gas, respectively. The two WO 3 doped TiO 2 sen- sors had a higher sensitivity to NO 2 than that of a pure TiO 2 sensor. Generally, the sensing film with a smaller grain size has a larger surface area of the sensing film and higher sensitivity [1]. There- fore, at 600 °C, the 15 wt.% WO 3 doped TiO 2 sensor had the highest sensitivity (145.4) due to its relatively small grain size. Due to its highest sensitivity to NO 2 , the 15 wt.% WO 3 doped TiO 2 sensor was used to verify the selectivity by detecting three other individual oxidizing gases, CO, O 2 and CO 2 , at 600 °C, and a gas mixture (NO 2 + oxidizing gases) with CO, O 2 and CO 2 concen- trations of 1000 ppm, 15% and 10%, respectively. The detection re- sults of the fabricated sensor for the various oxidizing gases and the gas mixture are shown in Figs. 6 and 7.InFig. 6, sensitivities of the sensor (15 wt.% WO 3 doped TiO 2 ) to CO, O 2 and CO 2 were very small. And in Fig. 7, the resistance of the sensor decreased very fast in the NO 2 condition but exhibited very small changes in the gas mixtures (NO 2 + other oxidizing gases). These sensing re- sults demonstrated the high sensitivity and good selectivity to NO 2 of the WO 3 doped TiO 2 sensor. The potential energy barrier eV S between particles of the sensor is proportional to N t , eV S ¼ e 2 N t 2 e 0 e r N d ð2Þ where N t is the surface density of adsorbed oxygen ions such as O À , O 2À ,O À 2 . And e r e 0 is the permittivity of the semiconductor, and N d the volumetric density of the electron donors. From Eq. (2),we can know that the sensitivity of the sensor is dominated by the number of adsorbed oxygen ions at the sensor surface. Because oxi- Fig. 4. XRD patterns – (1 1 0) peak of undoped TiO 2 , 11 wt.% WO 3 doped TiO 2 and 15 wt.% WO 3 doped TiO 2 . Fig. 5. The results of NO 2 (500 ppm) detection at 600 °C. Fig. 6. The results of CO, O 2 and CO 2 detection at 600 °C. S E. Jo et al. / Current Applied Physics xxx (2009) xxx–xxx 3 ARTICLE IN PRESS Please cite this article in press as: S E. Jo et al., Gas sensing properties of WO 3 doped rutile TiO 2 thick film at high operating temperature, Curr. Appl. Phys. (2009), doi:10.1016/j.cap.2009.0 6.053 dizing gases such as NO 2 are reacted with the oxygen species at the sensor surface as shown in Eq. (3) NO 2 ðgÞþO À 2 ! NO À 2 ðadsÞ: ð3Þ The presence of other metal oxides (WO 3 ) in the lattice of the TiO 2 film can cause a change of its electrical property and the inner morphological properties of the TiO 2 film, which made the smaller particle size of the sensor. It consequently affected the area of the sensor surface which related to the number of adsorbed oxygen ions and the gas sensing properties. Also, we assumed that the high heat treatment temperature of the sensor induced an electrical interaction between WO 3 and TiO 2 , which in turn affected the electrical properties of the sensing film. The WO 3 may occupy the interstitial sites in TiO 2 lattice according to the following: WO 3 ! TiO 2 W 00 Ti þ V 00 O þ O x O ð4Þ Since the predominant defect is an oxygen vacancy, this equilibrium can be described by Eq. (5), O x O ! 1 2 O 2 ðgÞþ2e 0 þ V 00 O ð5Þ where V 00 O is the oxygen vacancy and O x O is neutral oxygen in lattice. That is, WO 3 doping induces an increase of the oxygen partial pres- sure at the sensor surface. So, at high working temperature such as 600 °C, there is large number of adsorbed oxygen ions at the sur- face. And it makes the high surface density of adsorbed oxygen ions and high sensitivity to specific gas such as NO 2 . 4. Summary AWO 3 doped TiO 2 sensor, with a sensing film deposited by screen printing, was fabricated successfully. The use of rutile TiO 2 as the sensing film material enabled the heat treatment to be performed at high temperature, which facilitated the fabrication of a sensitive and porous shaped sensing film. The WO 3 doping did not exert a dominant effect on the surface topography but it did af- fect the inner morphological properties of the TiO 2 film. The 15 wt.% WO 3 doped TiO 2 sensor showed the best sensitivity to NO 2 at 600 °C. The proposed sensor had good sensitivity and selec- tivity to NO 2 , in comparison with its sensitivity to other oxidizing gases such as CO, O 2 , and CO 2 . Acknowledgements This work was supported in part by the Information Technology Research Center support program (IITA-2005-C1090-0592-0012), and in part by Korea Science and Engineering Foundation (KOSEF) Grant funded by the Korea government (MEST) (2008-8-1253). References [1] G. Korotcenkov, Materials Science and Engineering B 139 (2007) 1–23. [2] C.O. Park, S.A. Akbar, Journal of Materials Science 38 (2003) 4611–4637. [3] L. Francioso, D.S. Presicce, M. Epifani, P. Siciliano, A. Ficarella, Sensors and Actuators B 107 (2005) 563–571. [4] S.J. Kim, G.H. Chang, Y.C. Jin, G.R. Jheong, Journal of the Korean Society for Heat Treatment 7 (1) (1994) 11–16. [5] Ken-ichi Shimizu, Kohichi Kashiwagi, Hiroyuki Nishiyama, Shiro Kakimoto, Satoshi Sugaya, Hitoshi Yokoi, Atsushi Satsuma, Sensors and Actuators B 130 (2008) 707–712. [6] A. Wisitsoraat, E. Comini, G. Sberveglieri, W. Wlodarski, A. Tuantranont, in: The 2nd IEEE International Conference on Nano/Micro Engineered and Molecular Systems, 16–19th January 2007, Bangkok, Thailand. [7] S.S. Bhoga, K. Singh, Ionics 13 (2007) 417–427. Fig. 7. The results of gas mixture detection at 600 °C. 4 S E. Jo et al. / Current Applied Physics xxx (2009) xxx–xxx ARTICLE IN PRESS Please cite this article in press as: S E. Jo et al., Gas sensing properties of WO 3 doped rutile Ti O 2 thick film at high operating tempe rature, Curr. Appl. Phys. (2009), doi:10.1016/j.cap.2009.06.053 . TiO 2 based gas sensors can tolerate high heat treatment temperature levels. Therefore, rutile TiO 2 based gas sensors can detect gases at high operating temperature. the high heat treatment temperature ensures stable operation of the sensors at high temperature, it causes grain growth of the sensing films during the heat