Thin Solid Films 391 Ž2001. 255᎐260
Semiconductor gas sensors based on nanostructured tungsten oxide J.L. Solis a,1, S. Saukko b , L. Kish a,2 , C.G. Granqvist a , V. Lantto b,U a
˚ Department of Materials Science, The Angstrom ¨ Laboratory, Uppsala Uni¨ ersity, P.O. Box 534, Uppsala, SE-75121, Sweden b Microelectronics and Materials Physics Laboratories, Uni¨ ersity of Oulu, Linnanmaa, FIN-90570 Oulu, Finland
Abstract Semiconductor gas sensors based on nanocrystallline WO 3 films were produced by two different methods. Advanced reactive gas evaporation was used in both cases either for a direct deposition of films Ždeposited films. or to produce ultra fine WO 3 powder which was used for screen printing of thick films. The deposited films sintered at 480⬚C and the screen-printed films sintered at 500⬚C displayed a mixture of monoclinic and tetragonal phases and had a mean grain size of approximately 10 and 45 nm, respectively. We studied the influence of the sintering temperature Ts of the films on their gas sensitivity. Unique and excellent sensing properties were found upon exposure to low concentrations of H 2 S in air at room temperature for both deposited and screen-printed films sintered at Ts s 480⬚C and at Ts s 500⬚C, respectively. 䊚 2001 Elsevier Science B.V. All rights reserved. Keywords: Tungsten oxide; Gas deposition; Nanocrystalline; H 2 S sensing; Gas sensor
1. Introduction In the last decade, there has been an increasing interest in the study of nanocrystalline materials owing to their electrical, optical, mechanical and magnetic properties being superior to those of conventional coarse-grained structures w1᎐4x. The surface-to-bulk ratio for a nanocrystalline material is much greater than for a material with large grains, which yields a large interface between the solid and a gaseous or liquid medium. A chemical species on a ceramic semiconductor surface yields a signal that is transduced through the microstructure of the sintered ceramic to form a conductance change w5,6x. A discussion of the role of the size and shape of the contacts Žnecks.
U
Corresponding author: Tel.: q358-8-5532712; fax: q358-85532728. E-mail address:
[email protected] ŽV. Lantto.. 1 Permanent address: Facultad de Ciencias, Universidad Nacional de Ingenieria, P.O. Box 1301-Lima, Peru. 2 until 1999, L.B. Kiss.
between the grains for the transducer function is given by Yamazoe w5x. The interaction between a gas and a solid mainly takes place on the surface and hence the amount of atoms residing at grain surfaces and interfaces is critical for controlling the properties of the gas sensor. It is not uncommon that the portion of the surface atoms exceeds 50% in a nanocrystalline material w1᎐4x. Gas sensing applications of nanocrystalline materials have received considerable interest in recent years w7x and it is well known that the gas sensitivity of both porous SnO 2 w8x and WO 3 w9x films increases with decreasing grain size. Ceramic fabrication technology as well as thick- and thin-film processing of semiconducting oxides have been used during many years for conductance-based gas sensing w10x. There are only a few earlier studies of the gas sensing properties of nanocrystalline WO 3 films. In those cases the films were prepared by evaporation of tungsten in the presence of O 2 w11x, or by the sol᎐gel method w12x. In the present work we employ advanced reactive gas evaporation w13,14x both to deposit directly nanocrystalline WO 3 films Ždeposited films. and to
0040-6090r01r$ - see front matter 䊚 2001 Elsevier Science B.V. All rights reserved. PII: S 0 0 4 0 - 6 0 9 0 Ž 0 1 . 0 0 9 9 1 - 9
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produce ultra-fine particles ŽUFP. of WO 3 for screen printing of WO 3 thick films Žscreen-printed films.. Gas deposition ᎏ i.e. evaporation in the presence of a gas so that well-defined crystalline precursors for film manufacturing are formed ᎏ was based on induction heating of a tungsten pellet. Arc discharge vaporization of metal tungsten in a reactive atmosphere was used to produce UFP powder of WO 3 for screen printing. A drawback of the conventional semiconductor gas sensors is their operation at elevated temperatures Žtypically in the range 200 to 500⬚C., which implies that power is required. In recent years there has been a large effort to decrease the power consumption w15x and miniaturized low-power gas sensors have been developed with the sensitive layer on a micro-hotplate w16x. Obviously, it would be desirable for many applications if the sensor could operate at room temperature, and a reduction of the power consumption is a key goal for battery-operated devices. Recently, it has been reported that ZrO 2 ᎐SnO 2 w17x and ZnO w18x sensors can be used to detect H 2 S and NH 3 at room temperature, respectively, but their sensitivity was low. We report in this work that both deposited and screen-printed nanocrystalline WO 3 films sintered at 480⬚C and at 500⬚C, respectively, exhibit excellent sensing properties for small quantities of H 2 S in air even at room temperature. Structural studies gave some support to the tetragonal WO 3 phase, that is stabilized in the nanocrystalline WO 3 powder and films, being responsible for the unique gas sensing properties. 2. Experimental 2.1. Film deposition The deposited nanocrystalline WO 3 films were prepared using an advanced gas deposition unit ŽUltra Fine Particle Equipment, ULVAC Ltd., Japan.; this equipment was described elsewhere w19x. In essence, it comprises an evaporationrcondensation chamber, containing the starting material for the film, separated from a deposition chamber by a transfer pipe. Initially, the whole unit was evacuated to 3 = 10y2 mbar. Synthetic air Ž80 vol.% N2 and 20 vol.% O 2 . was then introduced into the evaporationrcondensation chamber to a pressure of 13 mbar. A highly laminar flow was created, which can lead to particle growth under nearequilibrium conditions with only a weak tendency towards agglomeration. The growth can be determined by first-passage-time dynamics in the vapor zone and the size distribution is narrow w20x. The starting material was a tungsten pellet Ž99.95%. positioned in the evaporationrcondensation chamber, where the heating and oxidation of the tungsten occurs. A surrounding copper coil inductively heats the pellet. During the deposition, the evaporation temperature was set to approximately
1100⬚C, as measured with an optical pyrometer. The pressure difference between the two chambers makes the formed particles go through the transfer pipe with the gas flow so that they are ejected out of a nozzle into the evacuated deposition chamber where they form a consolidated layer of tungsten oxide nanoparticles on an alumina substrate. The substrate was mounted on a table that can be scanned along the x, y and z-directions by a digital programmable controller; the scanning speed of the substrate was 1.5 mm sy1 . The alumina substrates had preprinted gold electrodes being 0.2 mm apart and a Pt heating resistor printed on the reverse side. Rectangular Ž3 = 2.5 mm2 . nanocrystalline WO 3 films with a thickness of 15 m were formed so they bridged the gold electrodes. Sintering of the films was carried out in air by heating at temperatures Ts in the 200 - Ts - 600⬚C range for 1 h. 2.2. Screen-printed films The ceramic UFP mode of the advanced gas evaporation unit referred to above was used to produce nanocrystalline WO 3 powder. The gas evaporation method uses vaporization of the material from a hearth followed by nucleation and particle growth in a gas stream. Atmospheric air was used as reactive and cooling gas. After evacuation of the UFP formation chamber, a N2 gas stream of 5 l miny1 and an atmospheric air stream of approximately 10 l miny1 were introduced. A d.c. power generator was connected between an anode and a water-cooled Cu hearth. A tungsten pellet Ž99.95%. was placed in the hearth and an arc discharge was generated between the tungsten and the anode by a current applied between them, thus producing the evaporation and oxidation of tungsten. The anode was moveable, and the arc discharge was ignited by a brief contact to the tungsten. The current between the anode and the tungsten pellet was kept fixed at 100 A. The WO 3 powder was collected in a separate chamber, which had a pressure difference to the ceramic UFP formation chamber. Thick-film pastes were prepared by adding 50 wt.% of an organic vehicle to 50 wt.% of the WO 3 nanopowder. After mixing the powder with the vehicle, the paste was milled in a triple mill in order to homogenize the mixture. The WO 3 thick films were then screen printed on alumina substrates. Rectangular Ž3 = 2.5 mm2 . nanocrystalline WO 3 films with a thickness of 7.5 m were printed onto the gold electrodes and the films were dried at 150⬚C for 0.5 h. Sintering of the films was carried out by heating at temperatures in the 300 - Ts - 800⬚C range. 2.3. Measurements The crystal structure and crystallite size of the nanocrystalline WO 3 films were determined by X-ray
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diffraction measurements with a Siemens D5000 diffractometer operating with CuK ␣ radiation and equipped with a Gobel mirror and a parallel plate ¨ collimator. The microstructures of the deposited films were analyzed by a scanning electron microscope ŽSEM., specifically a LEO 1550 instrument with a Gemini column and the screen-printed films were examined by a field-emission scanning electron microscope ŽFESEM. of the type JEOL JSM-6300F. Platinum wire contacts were attached with a lowtemperature gold paste to the two gold electrodes on the alumina substrate for electrical conductance measurements. The conductance of the films was obtained by measuring the current through the film at a constant voltage of 1 V. The samples under test were placed in a stainless steel chamber Ž500 cm3 . and exposed to different gas concentrations. Gas-sensing properties of the films were studied at various operating temperatures To in the 20 - To - 300⬚C range with a computer-controlled measuring system employing the flow-through principle w23x. H 2 S, H 2 , CO, NO, NO 2 and SO 2 at various concentrations in dry synthetic air were used as test gases in the measurements. 3. Structural properties The as-deposited nanocrystalline WO 3 films prepared by advanced reactive gas deposition were composed of crystallites with a tetragonal crystal structure and a mean grain size of ; 6 nm. The grain size was estimated from X-ray diffraction patterns using Scherrer’s formula w21x. This size is in agreement with earlier results w11x for WO 3 powder fabricated by gas evaporation. The screen-printed WO 3 films consisted of a mixture of both monoclinic and tetragonal phases in significant quantities and had a mean grain size of ; 40 nm. The tetragonal phase corresponds to the high temperature structure of WO 3 , stable above 770⬚C w22x. Clearly, the high temperature associated with the WO 3 evaporation during film fabrication can produce the tetragonal phase, which then stays metastable during cooling in the gas stream. Fig. 1 shows X-ray diffraction patterns obtained from deposited and screen-printed nanocrystalline WO 3 films sintered at 480⬚C and 500⬚C, respectively. Reflection peaks belonging to monoclinic as well as tetragonal phases of WO 3 are marked in the figure. The sharp peaks due to substrate are also indicated. Fig. 2a shows a SEM micrograph of the morphology of the deposited nanocrystalline WO 3 film sintered at 480⬚C. It exhibits a very porous structure with a grain size of approximately 15 nm. Fig. 2b shows a FESEM micrograph from the screen-printed WO 3 film sintered at 500⬚C. The microstructure of the film is seen to consist of large amount of small grains together with a
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Fig. 1. X-Ray diffraction patterns for both a deposited and a screenprinted nanocrystalline WO 3 film sintered at 480 and 500⬚C, respectively. Asterisks denote diffraction peaks from the substrate.
few large ones. Clearly, the SEM data are consistent with the information gained from X-ray diffraction. 4. Gas sensing properties The strength of the conductance response at exposure to a gas is described here by the conductance ratio
Fig. 2. Ža. SEM micrograph from a deposited nanocrystalline WO 3 film sintered at 480⬚C and Žb. FESEM micrograph from a screenprinted WO 3 film sintered at 500⬚C.
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GgasrGair , where Ggas and Gair denote the conductance in the test gas and in dry synthetic air, respectively. The ratio GgasrGair is used here also as a measure to describe the gas sensitivity of the samples. The sintering temperature was found to play an important role for the gas sensing properties. Fig. 3 shows the conductance response GgasrGair to H 2 S at room temperature of both deposited and screen-printed nanocrystalline WO 3 films sintered at different temperatures in the 300 - Ts - 800⬚C range. The films were subjected to 10 min of exposure to 10 ppm of H 2 S in synthetic air at To s 20⬚C. The conductance response GgasrGair of the deposited films increases with increasing Ts up to 480⬚C, whereas it decreases for higher Ts . On the other hand, the conductance response of the screen-printed films decreases with increasing Ts and is small at Ts ) 600⬚C. The conductance of the deposited film sintered at 480⬚C increased by approximately three orders of magnitude and that of the screen-printed film sintered at 300⬚C by more than four orders of magnitude. The conductance recovery was very slow and would take many hours to be complete. However, a short heat treatment Žapprox. 1 min. at 250⬚C after the exposure to H 2 S yielded a rapid conductance recovery to its initial value. Fig. 4 shows the conductance response GgasrGair of both deposited and a screen-printed WO 3 films sintered at the optimum temperatures of 480 and 500⬚C, respectively, to 10 ppm of H 2 S in synthetic air as a function of the operation temperature. The maximum response appears at room temperature for both types of films. The response of the deposited WO 3 film decreases exponentially with increasing To . At To ) 210⬚C, the ratio GgasrGair of both deposited and screen-printed films is only of the order of 10.
Fig. 4. Conductance response GgasrGair vs. operation temperature for deposited and a screen-printed nanocrystalline WO 3 films sintered at 480 and 500⬚C, respectively, exposed to 10 ppm of H 2 S in synthetic air.
Fig. 5 shows results of a more detailed study on the time dependence of room-temperature conductance, GŽ t .rGair , of a deposited WO 3 film sintered at 480⬚C and a screen-printed film sintered at 500⬚C during repeated exposures to increasing concentrations of H 2 S in synthetic air. Heating for a short time up to 250⬚C followed each exposure of the films to H 2 S. It is evident that the films were able to detect 1 ppm of H 2 S in synthetic air at room temperature during the time span of 10 min or more. Furthermore, it is found that the nanocrystalline WO 3 films recovered their initial conductance after the short annealing treatment at 250⬚C ŽFig. 5. irrespective of the H 2 S exposure. The nanocrystalline WO 3 films ᎏ both deposited and screen-printed ᎏ were not sensitive at room temperature to other tested gases, such as 100 ppm of CO, 10 ppm of NO, 500 ppm of H 2 , 100 ppm of SO 2 and 10 ppm NO 2 in synthetic air. Only the conductance of the screen-printed films sintered at 500⬚C decreased by factors of approximately 0.5 and 0.1 when exposed to 100 ppm of CO and 10 ppm of NO, respectively. 5. Discussion
Fig. 3. Conductance response GgasrGair vs. sintering temperature for deposited and screen-printed nanocrystalline WO 3 films exposed to 10 ppm of H 2 S in synthetic air at room temperature.
Our earlier X-ray diffraction studies of deposited WO 3 films in as-deposited form and after sintering at different temperatures up to 600⬚C showed a gradual phase transition to take place at Ts ) 400⬚C with a tetragonal phase changing to a monoclinic phase together with an increase of the grain size w24x. The tetragonal phase was practically absent in the films sintered at Ts ) 600⬚C and the grain size of the monoclinic phase increased to 78 nm for Ts s 700⬚C. The conductance response to H 2 S of the deposited films at room temperature was found to have a maximum after sintering at 480⬚C. Clearly, an ‘activation’
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grain-size effect in WO 3 films with grains smaller than 33 nm. The oxygen ions adsorbed at low temperatures on oxide semiconductor surfaces are thought to be Oy 2, even at room temperature w26x. Consequently, the electron transfer to surface species in connection with oxygen chemisorption creates a Schottky energy barrier at the surface yielding a low conductance of the film. In work by Fang et al. w17x, the H 2 S response was related to a catalytic reaction of H 2 S with the adsorbed Oy 2 ions. A release of electrons from the surface species decreases the height of the surface barrier, thereby resulting in an increase of the film conductance. In a model of Wang et al. w27x, a small grain size, such as in the deposited nanocrystalline WO 3 films after sintering at 480⬚C, improves the gas sensitivity. 6. Conclusions
Fig. 5. Conductance response vs. time, GŽ t .rGair , Ža. of a deposited WO 3 film sintered at 480⬚C and Žb. of a screen-printed WO 3 film sintered at 500⬚C at repeated exposures to different concentrations of H 2 S in synthetic air at room temperature. A temperature pulse up to 250⬚C follows each H 2 S exposure.
process was inherent in the sintering procedure. We note that a similar ‘activation’ to H 2 S was found in earlier work w25x on gold doped WO 3 films deposited by RF sputtering upon annealing them at 400⬚C. Our X-ray studies of the structure of the deposited WO 3 films sintered at 480⬚C showed that they consist of a mixture of tetragonal and monoclinic phases and the same is true for the screen-printed films sintered at 500⬚C. The sintering produces contacts between grains, many of which are between grains having different crystal structure. These grain contacts contribute significantly to the electrical conduction and presumably to the gas-sensing properties of the nanocrystalline WO 3 films. The conductance response to H 2 S at room temperature was found to almost disappear after sintering of the screen-printed WO 3 films above 600⬚C. Therefore the room temperature H 2 S sensitivity may be related to the presence of grains having a tetragonal phase in the films. We may note here that Tamaki et al. w9x found that the gas sensitivity was controlled by the
We prepared nanocrystalline WO 3 films with unique and excellent sensing properties upon exposure to low concentrations of H 2 S in air at room temperature. A tetragonal phase present in the WO 3 films may be responsible for their high room-temperature H 2 S sensitivity. The optimum sintering temperature for H 2 S sensing was found to be approximately 480⬚C and 500⬚C, respectively, for deposited and screen-printed WO 3 films. In addition to having a high H 2 S sensitivity, the films displayed a stable performance after short heating pulses at 250⬚C. The pulses were necessary to speed up the conductance recovery after H 2 S exposure. The nanocrystalline WO 3 films were able to detect 1 ppm of H 2 S in synthetic air at room temperature and did not have any noticeable cross sensitivity to CO, NO, H 2 , SO 2 and NO 2 . Acknowledgements This work was supported by Swedish Foundation for Strategic Research through its program on Advanced Micro Engineering and the Academy of Finland Žprojects 噛37778 and 噛44588.. References w1x w2x w3x w4x w5x w6x
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