Ž. Sensors and Actuators B 67 2000 270–274 www.elsevier.nlrlocatersensorb Gas sensing properties of metal-organics derived Pt dispersed-TiO thin 2 film fired in NH 3 I. Hayakawa a,) , Y. Iwamoto a,1 , K. Kikuta b , S. Hirano b a Fine Ceramics Research Association, Synergy Ceramics Laboratory, 2-4-1, Mutsuno, Atsuta-ku, Nagoya, 456-8587, Japan b Graduate School of Engineering, Nagoya UniÕersity, Nagoya, 464-8603, Japan Received 20 December 1999; received in revised form 23 April 2000; accepted 25 April 2000 Abstract Metal-organic precursor solution for coating was synthesized using Ti alkoxide derivative, amino acid, platinum salt and methanol as a solvent, in which TiO sol was also added to control the pore structure. This solution was spin coated on glass substrate and pretreated in 2 wet air, followed by firing in 3% H rAr. The thin film fired at 4508C showed the highest gas sensitivity and selectivity to H . However, 2 2 the film fired at 6008C showed no sensitivity to reducing gases. In contrast, high gas sensitivity and selectivity to H was observed on the 2 film fired in NH at 6008C, in which the solid solution of nitrogen into TiO was observed. The firing in NH is considered to suppress 3 23 the degradation of sensitivity resulting from SMSI. q 2000 Elsevier Science S.A. All rights reserved. Keywords: Sensor; Thin film; TiO ; Platinum; NH ; SMSI; Metal-organics; TiO sol 23 2 1. Introduction A great deal of efforts has been put into developing new sensing materials with improved sensor properties. Of these, n-type semiconducting materials such as SnO , ZnO 2 wx and TiO are promising materials for gas sensor 1 . 2 TiO has been mainly studied as a material of O 22 sensor at high temperature as high as 8008C in the form of wx bulk or thick film 2,3 . However, there is little trial to develop TiO -based thin film to detect a gas at low 2 temperature, because gas sensitivity of TiO is quite low 2 compared with that of SnO that has commonly been used. 2 A salt of noble metal is sometimes added to a sensor material for the purpose of improving gas sensitivity. A TiO -based sensor material added with a noble metal salt 2 is generally fired in a reducing atmosphere to form fine metal particles or to offer n-type semiconductivity, which contributes to supply electrons necessary for adsorption of oxygen. However, it is known in the field of catalyst that ) Corresponding author. Present address: Planning Department, Corpo- rate Research and Development, Group, NGK Insulators, Ltd., Nagoya, Japan. 1 Present address: Darmstadt University of Technology, Darmstadt, Germany. the degradation of catalytic activity happens in the system Ž TiO -noble metal: especially Pt, by SMSI Strong Metal 2 . Substrate Interaction effect when it was heated in H 2 wx atmosphere above 5008C 4–9 . In these papers, SMSI is explained by the effect of encapsulation or decoration of the metal by the reduced support or electronic interaction of the reduced support with the metal. SMSI decreases the adsorption of H or CO on the metal particle. This will 2 decrease the reactivity of O adsorbed on the metal with 2 H . Therefore, TiO –Pt with SMSI will not greatly change 22 the resistance when H was introduced. 2 NH is a strong reducing gas because hydrogen pro- 3 duced by the decomposition exerts the high reduction wx effect on TiO 10 . Also, nitrogen produced at the same 2 time reacts with oxide to form a solid solution or a nitride wx 11 . Formation of Ti–N bonds is considered to affect the activity of Pt that is related to gas sensitivity. It can be suggested that the use of a metal-organic precursor as a starting material is very effective to improve gas sensitivity and selectivity at low temperatures because the obtained microstructure contains very fine TiO grains 2 wx and finely dispersed Pt particles 12 . It is possible to form films with controlled microstructure in nanoscale since each element is homogeneously mixed and bonded at molecular level in precursor solution. Therefore, it is con- sidered that NH affects this material more effectively. 3 0925-4005r00r$ - see front matter q 2000 Elsevier Science S.A. All rights reserved. Ž. PII: S0925-4005 00 00517-7 () I. Hayakawa et al.rSensors and Actuators B 67 2000 270–274 271 Fig. 1. Change of gas sensitivity with firing temperature. This paper described the effect of NH on gas sensing 3 properties of Pt dispersed-TiO derived from precursor. 2 2. Experimental Precursor solution for coating was prepared as follows. Ž.Ž. A 75% isopropanol solution of Ti O-iPr AcAc , Nisso: 22 T-50, was used as a Ti source. Methanol solution of L-Lysine was reacted with that of T-50. Platinum salt, H PtCl P 6H O, dissolved in methanol was then reacted 26 2 with this reacted solution. A metal-organic compound containing Ti and Pt elements in the same molecule was synthesized by this process, which used L-lysine as a linking medium of Ti and Pt. Then, an excess amount of water was added to hydrolyze the residual alkoxy groups Ž.Ž. of Ti O-iPr AcAc . The amount of platinum salt was 22 adjusted to the composition of 2 wt.% Pt in TiO matrix. 2 The TiO sol was added to the synthesized solution with a 2 composition of 50 wt.% as TiO to form many fine pores 2 wx in the resultant thin film 12 . Then, the mixed solution was homogeneously dispersed by ultrasonicaction. TiO 2 Ž. particles in TiO sol, STS-02 Ishihara Sangyo are 7 nm 2 in primary particle size and are stabilized in suspension by acid. Moreover, the coating solution without Pt was pre- pared as a reference by the same method without adding Pt salt. The coating solutions with and without Pt were spin Ž. coated on corning glass a7059 substrates. A spin coating was done for 20 s at 2000 rpm. The coated precursor films were dried for 1 day at r.t. in air and preheated at 4008Cin wet air under atmospheric pressure to hydrolyze com- pletely and to eliminate organic components. Then, the preheated films were fired at 4008C–6008Cin3%HrAr, 2 at 6008C–6508CinNH orat6008C in Ar with Ti. Ti was 3 used to eliminate oxygen in Ar. The thickness of the thin films was about 70 nm. The gas sensitivity of the thin film with Ag electrode was almost the same as that with Au electrode. However, Au electrode was easily torn from thin film. Therefore, Ag electrode was adopted. Ag electrodes were formed by printing Ag paste on the thin films with the spacing of 1 mm between two electrodes. The thin films were mounted on a guard electrode to decrease current through a glass substrate. A voltage of5VinDCwasimposed between Ž two electrodes under flow of several kinds of gases 1000 . ppmrair at 1708C–2308C. A flow rate was 200 mlrmin and controlled by a mass flow meter. Current between two electrodes was measured by a picoammeter and was auto- matically converted into the value of resistance. In this paper, the gas sensitivity was defined as the ratio of Ž. Ž. resistance Ro in air to that R in a sample gas using the wx same equation as described by Egashira et al. 13 . Crystalline phases in thin films were analyzed by means Ž. of XRD X-ray Diffraction . Microstructures of some thin Ž films were observed with TEM transmission electron .Ž microscope and FE-SEM field emission-scanning elec- . tron microscope . Valence states of Ti and Pt were Ž examined by ESCA electron spectroscopy for chemical . analysis , and chemical compositions of thin films were Ž. analyzed by SIMS secondary ion mass spectroscopy . 3. Results and discussion The spin coated thin films were preheated at 4008Cin wet air and fired at 4008C–5508C under 3% H rAr. The 2 gas sensitivity at 2008C is shown in Fig. 1 as a function of firing temperature. The gas sensitivities to 1000 ppm CO and CH were very low and independent of the firing 4 temperature in the range of 4008C–5508C. In contrast, the gas sensitivity to 1000 ppm H greatly depended on the 2 firing temperature. The sensitivity became the maximum on the film fired at 4508C. The thin film fired at 4508C proved to have the highest gas sensitivity and selectivity to H among reducing gases: H , CO and CH . A XRD 224 profile of this thin film indicated the presence of only anatase phase. In contrast, the sensitivities of the films fired at 5008C and 5508C remarkably decreased in compar- ison with that at 4508C. Microstructure and crystalline Table 1 Ž Gas sensitivity of the thin film fired in different atmosphere measured at . 2008C Firing atmosphere 1000 ppm H 1000 ppm CO 1000 ppm CH 24 NH 134 0.8 1.9 3 3%H rAr 2.1 0.7 1.1 2 Ž. Ar Ti 2.0 0.8 1.0 () I. Hayakawa et al.rSensors and Actuators B 67 2000 270–274272 Table 2 Ž. Relative resistance of the thin film fired in different atmosphere measured at 2008C Number Firing atmosphere Kind of thin film Kind of measuring gas Relative resistance 1a NH TiO –Pt air 850 32 1H NH TiO –Pt 1000 ppm H 6.5 32 2 2a NH TiO air 26 32 2H NH TiO 1000 ppm H 22 32 2 Ž. 3a 3%H rAr TiO –Pt air 1 unit 22 3H 3%H rAr TiO –Pt 1000 ppm H 1.6 22 2 4a 3%H rAr TiO air 1.4 22 4H 3%H rAr TiO 1000 ppm H 1.1 22 2 phase of the thin film fired at 4508C were compared with those at 5008C. Both thin films consisted of only anatase phase and showed almost the same XRD profiles, grain size of TiO and pore structures important to sensitivity. 2 The grain size of TiO was about 10 nm from TEM 2 observation. Therefore, the decrease of the sensitivity of the films fired above 5008C is considered to be due to the SMSI effect. Sensitivity to various gases measured at 2008C is shown for the thin films fired in NH , 3% H rAr or Ar at 6008C 32 in Table 1. High sensitivity and selectivity to H was 2 observed at the film fired in NH . However, the film fired 3 in 3% H rAr or Ar was not sensitive to H , CO and CH . 224 Also, the films without Pt did not show the sensitivity to gas irrespective of firing atmosphere. Table 2 shows the relative resistance of the thin films fired in NH or 3% H rAr when the relative resistance of 32 the TiO –Pt film fired in 3% H rAr is unit. The sensitiv- 22 ity to H is expressed by the ratio of the resistance in air 2 to that in 1000 ppm H . First, in the case of the thin film 2 fired in NH , the resistance of the TiO –Pt film was 32 compared with that of the TiO film. Although the resis- 2 tance of 1H is smaller than that of 2H, the resistance of 1a is larger than that of 2a. It turns out that the TiO –Pt film 2 is a little more reduced than the TiO film, but the 2 resistance of the TiO –Pt film in air becomes extremely 2 high compared with that of the TiO film because elec- 2 trons in TiO transfer to oxygen adsorbed on active Pt in 2 the TiO –Pt film. 2 In the case of the TiO –Pt film, the resistance of the 2 film fired in NH was compared with that in 3% H rAr. 32 Although the resistance of 3H is the same order of magni- tude as that of 1H, the resistance of 3a is extremely lower than that of 1a. This means that the film fired in 3% H rAr is reduced to the same level as that in NH , but the 23 resistance becomes extremely low in air because the elec- tron transfer derived from the adsorption of oxygen does not occur in this film. Characterization was performed for the films fired in NH , 3% H rAr or Ar. No difference was observed as to 32 the microstructure of thin film, namely, grain size and pore structure. XRD showed that each TiO –Pt film was com- 2 posed of only anatase phase, and has almost the same crystallinity. In contrast, the TiO film fired in NH 23 showed poor crystallinity compared with that in 3% H rAr 2 as shown in Fig. 2. It was presumed that the firing atmosphere under the existence of Pt did not affect the crystallinity of TiO . The ESCA profiles for the film fired 2 0 Ž. in 3% H rAr indicated that Pt exists as Pt metal and 2 Pt 2q , and Ti as almost all Ti 4q . Fig. 3 shows the SIMS profiles of the thin films fired in 3% H rAr or NH . 23 Ž. Concentration of N nitrogen was higher by one order of magnitude in the film fired in NH than in 3% H rAr. 32 This implies that the nitrogen produced by the decomposi- tion of NH diffuses into thin films and forms Ti–N 3 bonds. Moreover, the pretreated thin films were fired at 6258C or 6508C in NH . These films showed the remarkably low 3 resistance compared with that of 6008C and only a slight sensitivity to H . However, the annealing at 3008C–3508C 2 in air increased the resistance of the films and recovered the sensitivity. The sensitivity to H increased with the 2 increasing resistance accompanied by annealing as shown in Fig. 4. The measuring temperature giving a maximum value of sensitivity changed between 1708C and 2308C depending on the firing and annealing temperature. There- fore, Fig. 4 contains data of 1708C to 2308C. The anneal- ing at 3008C–3508C did not affect the grain size and crystalline phase of TiO and pore structure of the thin 2 film. Hence, the sensitivity is considered to depend mainly Fig. 2. XRD profiles for the TiO thin film fired in 3% H rAr or NH . 223 () I. Hayakawa et al.rSensors and Actuators B 67 2000 270–274 273 Fig. 3. SIMS profiles for the TiO –Pt thin film fired in 3% H rAr or NH . 223 on the properties of platinum particles. The low sensitivity and resistance of the as-fired film and the recovery of the sensitivity by annealing indicates that the SMSI occurred in the films fired in NH at 6258C and 6508C. Also, this 3 figure reveals that the film fired in NH has higher gas 3 sensitivity than that in 3% H rAr at the same resistance. 2 This means that the effects of firing atmosphere on proper- ties of Pt particles remarkably differs between NH and 3 3%H rAr. The firing in NH is effective to suppress the 23 degradation of sensibility resulting from the SMSI. Further analysis is necessary to clarify the effects of firing atmo- sphere on the properties of platinum. The degradation of sensibility was observed above 5008C in the film fired in 3% H rAr and above 6258Cin 2 the film fired in NH . The increase of the temperature at 3 which the degradation occurs is advantageous to doping of other metal component and control of microstructure or Fig. 4. Sensitivity and resistance of TiO –Pt thin film fired in 3% 2 H rAr or NH . 23 crystalline phase in the noble metal-TiO based material, 2 aiming at the development of sensor or catalyst. 4. Conclusion The precursor solution for coating was successfully synthesized using Ti alkoxide derivative, amino acid, plat- inum salt, methanol as a solvent, and TiO sol to control 2 the pore structure. The thin film coated with the precursor solution was fired in 3% H rAr or NH , etc., and the gas 23 sensing properties were compared. The thin film fired in 3% H rAr at 6008C showed no sensitivity to reducing 2 gases. In contrast, the highest gas sensitivity and selectiv- ity to H was observed for the film fired in NH at 6008C. 23 The temperature at which degradation of sensibility occurs was higher by about 1308C in NH than in 3% H rAr. 32 The firing in NH is effective to suppress the degradation 3 of sensibility. Acknowledgements Work supported by NEDO as part of the Synergy Ceramics Project under the International Science and tech- Ž. nology Frontier ISTF Program promoted by AIST, MITI, Japan. The authors, I. Hayakawa and Y. Iwamoto, were members of the Joint Research Consortium of Synergy Ceramics until March in 1999. References wx 1 C.N.R. Rao, A.R. Raju, K. Vijayamohanan, Gas-sensor materials, Ž. discussion-meeting on new materials: Bangalore, New Mater. 1992 1–37. wx 2 A. Takami, Development of titania heated exhaust-gas oxygen sen- Ž. sor, Ceram. Bull. 67 1988 1956–1960. wx Ž. 3 T. Takeuchi, Oxygen sensors, Sens. Actuators 14 1988 109–124. () I. Hayakawa et al.rSensors and Actuators B 67 2000 270–274274 wx 4 R.T.K. Baker, E.B. Prestridge, R.L. Garten, Electron microscopy of supported metal particles: 1. Behavior of Pt on titanium oxide, Ž. aluminum oxides, silicon oxide and carbon, J. Catal. 56 1979 390–406. wx 5 R.T.K. Baker, E.B. Prestridge, R.L. Garten, Electron microscopy of supported metal particles: II. Further studies of PtrTiO , J. Catal. 2 Ž. 59 1979 293–302. wx 6 S.J. Tauster, S.C. Fung, R.L. Garten, Strong metal-support interac- tions, group 8 noble metals supported on TiO , J. Am. Chem. Soc. 2 Ž.Ž . 100 1 1978 170–175. wx 7 G.B. Hoflund, A.L. Grogan Jr., D.A. Asbury, An ISS, AES, and ESCA study of the oxidative and reductive properties of platinized Ž. titania, Catalysis 109 1988 226–231. wx 8 D.N. Belton, Y M. Sun, J.M. White, Metal-support interaction on Ž. Rh and PtrTiO model catalysts, J. Phys. Chem. 88 1984 5172– 2 7176. wx 9 J.M. Herrmann, M. Gravelle-Rumeau-Maillot, P.C. Gravelle, A microcalorimetric study of metal-support interaction in the PtrTiO 2 Ž. system, J. Catal. 104 1987 136–146. wx 10 S. Kobayashi, D. Shibuta, K. Yajima, F. Suzuki, Functional su- Ž.Ž . perfine powder: titanium black, Kogyo Zairyo 32 13 1984 50–55. wx Ž. 11 D. Shibuta, Y. Sakai, M. Yoshizumi, in: G.L. Messing et al. eds. , Ceramic Transactions, Ceramic Powder Science 2, vol. 1, The American Ceramic Society, Westerville, 1988, pp. 848–855. wx 12 I. Hayakawa, Y. Iwamoto, K. Kikuta, S. Hirano et al., Gas sensing properties of platinum dispersed-TiO thin film derived from precur- 2 Ž. sor, Sens. Actuators, B 62 2000 55–60. wx 13 M. Egashira, Y. Shimizu, Y. Takao, Enhancement of trimethylamine sensitivity of semiconductor gas sensors by ruthenium, Chem. Lett. Ž. 1988 389–392. Biographies Issei Hayakawa received his B.S. in 1973 from Nagoya University, M.S. in 1975 from the University of Tokyo and Dr. Eng. degree in 1992 from Kyushu University. He has been engaged in development of new ceramic materials and new manufacturing processes at NGK Insulators since 1975. He studied the synthesis and evaluation of thin films derived from metal-organic precursors, aiming at development of new sensing materi- als and catalyst, under Synergy Ceramics Project. He currently belongs to NGK Insulators. Yuji Iwamoto received his B.S. and M.S. degrees in organic chemistry from the Faculty of Pharmaceutical Science, Nagoya City University in 1985 and 1987, respectively. He studied the design and synthesis of metal-organic precursor for ceramic materials under Synergy Ceramics Project. He has been currently sent to Darmstadt University of Technol- ogy to do research. Ko-ichi Kikuta received his M. Eng. and Dr. Eng. degrees in applied chemistry from Nagoya University in 1986 and 1989. He is currently an associate professor in the Department of Crystalline Materials Science, Nagoya University. His research interests include chemical processing of functional materials and composites. Shin-ichi Hirano received his B.S., M.S. and Dr. Eng. degrees in applied chemistry from Nagoya University in 1965, 1967 and 1970, respectively. He is currently a professor in the Department of Applied Chemistry, Nagoya University. His research interests include chemical processing of functional ceramics and inorganicrorganic hybrids, and in-situ mi- crostructural control of ceramic composites.