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
Trang 1I Hayakawaa,), Y Iwamotoa,1, K Kikutab, S Hiranob
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 in2 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 the2 film fired in NH at 6008C, in which the solid solution of nitrogen into TiO was observed The firing in NH is considered to suppress3 2 3 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 sol2 3 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 , ZnO2
w x and TiO are promising materials for gas sensor 1 2
TiO2 has been mainly studied as a material of O2
sensor at high temperature as high as 8008C in the form of
w x bulk or thick film 2,3 However, there is little trial to
develop TiO -based thin film to detect a gas at low2
temperature, because gas sensitivity of TiO is quite low2
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 salt2
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 Metal2
Substrate Interaction effect when it was heated in H2
w x 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 H2 or CO on the metal particle This will decrease the reactivity of O adsorbed on the metal with2
H Therefore, TiO –Pt with SMSI will not greatly change2 2 the resistance when H was introduced.2
NH3 is a strong reducing gas because hydrogen pro-duced by the decomposition exerts the high reduction
w x effect on TiO2 10 Also, nitrogen produced at the same time reacts with oxide to form a solid solution or a nitride
w11 Formation of Ti–N bonds is considered to affect thex 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 grains2
w x 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.
PII: S 0 9 2 5 - 4 0 0 5 0 0 0 0 5 1 7 - 7
Trang 2Fig 1 Change of gas sensitivity with firing temperature.
This paper described the effect of NH on gas sensing3
properties of Pt dispersed-TiO derived from precursor.2
2 Experimental
Precursor solution for coating was prepared as follows
A 75% isopropanol solution of Ti O-iPr 2 AcAc 2, Nisso:
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 reacted2 6 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
of Ti O-iPr 2 AcAc The amount of platinum salt was2
adjusted to the composition of 2 wt.% Pt in TiO matrix.2
The TiO sol was added to the synthesized solution with a2
composition of 50 wt.% as TiO to form many fine pores2
w x
in the resultant thin film 12 Then, the mixed solution
was homogeneously dispersed by ultrasonicaction TiO2
particles in TiO sol, STS-02 Ishihara Sangyo are 7 nm2
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
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 4008C in
wet air under atmospheric pressure to hydrolyze
com-pletely and to eliminate organic components Then, the
preheated films were fired at 4008C–6008C in 3% H rAr,2
at 6008C–6508C in NH or at 6008C in Ar with Ti Ti was
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 of 5 V in DC was imposed 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
resistance Ro in air to that R in a sample gas using the
w x
same equation as described by Egashira et al 13
Crystalline phases in thin films were analyzed by means
of XRD X-ray Diffraction Microstructures of some thin
Ž films were observed with TEM transmission electron
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
analyzed by SIMS secondary ion mass spectroscopy
3 Results and discussion
The spin coated thin films were preheated at 4008C in wet air and fired at 4008C–5508C under 3% H rAr The2 gas sensitivity at 2008C is shown in Fig 1 as a function of firing temperature The gas sensitivities to 1000 ppm CO and CH4 were very low and independent of the firing temperature in the range of 4008C–5508C In contrast, the gas sensitivity to 1000 ppm H2 greatly depended on the 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
H2 among reducing gases: H , CO and CH A XRD2 4 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 H2 1000 ppm CO 1000 ppm CH4
Ž
Trang 3Table 2
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
Ž
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 TiO2 was about 10 nm from TEM
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 6008C3 2
in Table 1 High sensitivity and selectivity to H2 was
observed at the film fired in NH However, the film fired3
in 3% H rAr or Ar was not sensitive to H , CO and CH 2 2 4
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 of3 2
the TiO –Pt film fired in 3% H rAr is unit The sensitiv-2 2
ity to H2 is expressed by the ratio of the resistance in air
to that in 1000 ppm H First, in the case of the thin film2
fired in NH , the resistance of the TiO –Pt film was3 2
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 film2
is a little more reduced than the TiO2 film, but the
resistance of the TiO –Pt film in air becomes extremely2
high compared with that of the TiO2 film because
elec-trons in TiO transfer to oxygen adsorbed on active Pt in2
the TiO –Pt film.2
In the case of the TiO –Pt film, the resistance of the2
film fired in NH was compared with that in 3% H rAr.3 2
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 the2 3
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 to3 2
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 TiO2 film fired in NH3 showed poor crystallinity compared with that in 3% H rAr2
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 fired2
0 Ž
in 3% H rAr indicated that Pt exists as Pt2 metal and
Pt2q, and Ti as almost all Ti4q Fig 3 shows the SIMS profiles of the thin films fired in 3% H rAr or NH 2 3
Concentration of N nitrogen was higher by one order of magnitude in the film fired in NH3 than in 3% H rAr.2 This implies that the nitrogen produced by the decomposi-tion of NH3 diffuses into thin films and forms Ti–N bonds
Moreover, the pretreated thin films were fired at 6258C
or 6508C in NH These films showed the remarkably low3 resistance compared with that of 6008C and only a slight sensitivity to H However, the annealing at 3008C–3508C2
in air increased the resistance of the films and recovered the sensitivity The sensitivity to H2 increased with the 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 TiO2 and pore structure of the thin 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
Trang 4Fig 3 SIMS profiles for the TiO –Pt thin film fired in 3% H rAr or NH 2 2 3
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, this3
figure reveals that the film fired in NH3 has higher gas
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 NH3 and
3%H rAr The firing in NH is effective to suppress the2 3
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 6258C in2
the film fired in NH The increase of the temperature at3
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
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 control2 the pore structure The thin film coated with the precursor solution was fired in 3% H rAr or NH , etc., and the gas2 3 sensing properties were compared The thin film fired in 3% H rAr at 6008C showed no sensitivity to reducing2 gases In contrast, the highest gas sensitivity and selectiv-ity to H was observed for the film fired in NH at 6008C.2 3 The temperature at which degradation of sensibility occurs was higher by about 1308C in NH3 than in 3% H rAr.2 The firing in NH is effective to suppress the degradation3
of sensibility
Acknowledgements
Work supported by NEDO as part of the Synergy Ceramics Project under the International Science and
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
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.