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.

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

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-

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

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
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

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.

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.
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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