semiconductor gas sensors based on nanostructured tungsten oxide

Ž. Thin Solid Films 391 2001 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 films were produced by two different methods. Advanced reactive 3 Ž. gas evaporation was used in both cases either for a direct deposition of films deposited films or to produce ultra fine WO 3 powder which was used for screen printing of thick films. The deposited films sintered at 480ЊC and the screen-printed films sintered at 500ЊC 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 T of the films on their gas sensitivity. Unique and s excellent sensing properties were found upon exposure to low concentrations of H S in air at room temperature for both 2 deposited and screen-printed films sintered at T s 480ЊC and at T s500ЊC, respectively. ᮊ 2001 Elsevier Science B.V. All rights ss reserved. Keywords: Tungsten oxide; Gas deposition; Nanocrystalline; H S sensing; Gas sensor 2 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 wx coarse-grained structures 1᎐4 . 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 semi- conductor surface yields a signal that is transduced through the microstructure of the sintered ceramic to wx form a conductance change 5,6 . A discussion of the Ž. role of the size and shape of the contacts necks U Corresponding author: Tel.: q358-8-5532712; fax: q358-8- 5532728. Ž. E-mail address: vila@ee.oulu.fi V. 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 wx by Yamazoe 5 . 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 inter- faces 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 mate- wx rial 1᎐4 . Gas sensing applications of nanocrystalline materials have received considerable interest in recent wx years 7 and it is well known that the gas sensitivity of wx wx both porous SnO 8 and WO 9 films increases with 23 decreasing grain size. Ceramic fabrication technology as well as thick- and thin-film processing of semicon- ducting oxides have been used during many years for wx conductance-based gas sensing 10 . There are only a few earlier studies of the gas sensing properties of nanocrystalline WO films. In 3 those cases the films were prepared by evaporation of wx tungsten in the presence of O 11 , or by the sol᎐gel 2 wx method 12 . In the present work we employ advanced wx reactive gas evaporation 13,14 both to deposit directly Ž. nanocrystalline WO films deposited films and to 3 0040-6090r01r$ - see front matter ᮊ 2001 Elsevier Science B.V. All rights reserved. Ž. PII: S 0 0 4 0 - 6 0 9 0 0 1 00991-9 () J.L. Solis et al. r Thin Solid Films 391 2001 255᎐260256 Ž. produce ultra-fine particles UFP of WO for screen 3 Ž. printing of WO thick films screen-printed films . Gas 3 deposition ᎏ i.e. evaporation in the presence of a gas so that well-defined crystalline precursors for film man- ufacturing are formed ᎏ was based on induction heat- ing of a tungsten pellet. Arc discharge vaporization of metal tungsten in a reactive atmosphere was used to produce UFP powder of WO for screen printing. 3 A drawback of the conventional semiconductor gas sensors is their operation at elevated temperatures Ž. typically in the range 200 to 500ЊC , which implies that power is required. In recent years there has been a wx large effort to decrease the power consumption 15 and miniaturized low-power gas sensors have been developed with the sensitive layer on a micro-hotplate wx 16 . Obviously, it would be desirable for many applica- tions 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 re- wx wx ported that ZrO ᎐SnO 17 and ZnO 18 sensors can 22 be used to detect H S and NH at room temperature, 23 respectively, but their sensitivity was low. We report in this work that both deposited and screen-printed nanocrystalline WO films sintered at 3 480ЊC and at 500ЊC, respectively, exhibit excellent sens- ing properties for small quantities of H S in air even at 2 room temperature. Structural studies gave some sup- port to the tetragonal WO phase, that is stabilized in 3 the nanocrystalline WO powder and films, being re- 3 sponsible for the unique gas sensing properties. 2. Experimental 2.1. Film deposition The deposited nanocrystalline WO films were pre- 3 Ž pared using an advanced gas deposition unit Ultra . Fine Particle Equipment, ULVAC Ltd., Japan ; this wx equipment was described elsewhere 19 . In essence, it comprises an evaporationrcondensation chamber, con- taining the starting material for the film, separated from a deposition chamber by a transfer pipe. Initially, the whole unit was evacuated to 3= 10 y2 mbar. Syn- Ž. thetic air 80 vol.% N and 20 vol.% O was then 22 introduced into the evaporationrcondensation cham- ber to a pressure of 13 mbar. A highly laminar flow was created, which can lead to particle growth under near- equilibrium conditions with only a weak tendency to- wards agglomeration. The growth can be determined by first-passage-time dynamics in the vapor zone and the wx size distribution is narrow 20 . The starting material Ž. was a tungsten pellet 99.95% positioned in the evap- orationrcondensation 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 1100ЊC, 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-direc- tions by a digital programmable controller; the scan- ning speed of the substrate was 1.5 mm s y1 . The alumina substrates had preprinted gold electrodes be- ing 0.2 mm apart and a Pt heating resistor printed on Ž 2 . the reverse side. Rectangular 3= 2.5 mm nanocrys- talline WO films with a thickness of 15 ␮m were 3 formed so they bridged the gold electrodes. Sintering of the films was carried out in air by heating at tempera- tures T in the 200- T - 600ЊC range for 1 h. ss 2.2. Screen-printed films The ceramic UFP mode of the advanced gas evap- oration unit referred to above was used to produce nanocrystalline WO powder. The gas evaporation 3 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 N gas stream of 5 l min y1 and an atmo- 2 spheric air stream of approximately 10 l min y1 were introduced. A d.c. power generator was connected between an anode and a water-cooled Cu hearth. A Ž. tungsten pellet 99.95% 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 powder was collected in a 3 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 homog- enize the mixture. The WO thick films were then 3 Ž screen printed on alumina substrates. Rectangular 3= 2 . 2.5 mm nanocrystalline WO films with a thickness of 3 7.5 ␮m were printed onto the gold electrodes and the films were dried at 150ЊC for 0.5 h. Sintering of the films was carried out by heating at temperatures in the 300- T - 800ЊC range. s 2.3. Measurements The crystal structure and crystallite size of the nanocrystalline WO films were determined by X-ray 3 () J.L. Solis et al. r Thin Solid Films 391 2001 255᎐260 257 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 Ž. SEM , specifically a LEO 1550 instrument with a Gemini column and the screen-printed films were ex- amined by a field-emission scanning electron micro- Ž. scope FESEM of the type JEOL JSM-6300F. Platinum wire contacts were attached with a low- temperature gold paste to the two gold electrodes on the alumina substrate for electrical conductance mea- surements. 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 Ž 3 . stainless steel chamber 500 cm and exposed to dif- ferent gas concentrations. Gas-sensing properties of the films were studied at various operating tempera- tures T in the 20 - T - 300ЊC range with a oo computer-controlled measuring system employing the wx flow-through principle 23 . H S, H , CO, NO, NO 22 2 and SO at various concentrations in dry synthetic air 2 were used as test gases in the measurements. 3. Structural properties The as-deposited nanocrystalline WO films pre- 3 pared by advanced reactive gas deposition were com- posed 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 Scher- wx rer’s formula 21 . This size is in agreement with earlier wx results 11 for WO powder fabricated by gas evapora- 3 tion. The screen-printed WO films consisted of a 3 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 wx temperature structure of WO , stable above 770ЊC22. 3 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 films 3 sintered at 480ЊC and 500ЊC, respectively. Reflection peaks belonging to monoclinic as well as tetragonal phases of WO are marked in the figure. The sharp 3 peaks due to substrate are also indicated. Fig. 2a shows a SEM micrograph of the morphology of the deposited nanocrystalline WO film sintered at 3 480ЊC. 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 film sintered 3 at 500ЊC. The microstructure of the film is seen to consist of large amount of small grains together with a Fig. 1. X-Ray diffraction patterns for both a deposited and a screen- printed nanocrystalline WO film sintered at 480 and 500ЊC, respec- 3 tively. 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 expo- sure to a gas is described here by the conductance ratio Ž. Fig. 2. a SEM micrograph from a deposited nanocrystalline WO 3 Ž. film sintered at 480ЊC and b FESEM micrograph from a screen- printed WO film sintered at 500ЊC. 3 () J.L. Solis et al. r Thin Solid Films 391 2001 255᎐260258 G rG , where G and G denote the conductance gas air gas air in the test gas and in dry synthetic air, respectively. The ratio G rG is used here also as a measure to gas air describe the gas sensitivity of the samples. The sinter- ing temperature was found to play an important role for the gas sensing properties. Fig. 3 shows the conduc- tance response G rG to H S at room temperature gas air 2 of both deposited and screen-printed nanocrystalline WO films sintered at different temperatures in the 3 300- T - 800ЊC range. The films were subjected to 10 s min of exposure to 10 ppm of H S in synthetic air at 2 T s 20ЊC. The conductance response G rG of the o gas air deposited films increases with increasing T up to 480ЊC, s whereas it decreases for higher T . On the other hand, s the conductance response of the screen-printed films decreases with increasing T and is small at T ) 600ЊC. ss The conductance of the deposited film sintered at 480ЊC increased by approximately three orders of mag- nitude and that of the screen-printed film sintered at 300ЊC 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 Ž. treatment approx. 1 min at 250ЊC after the exposure to H S yielded a rapid conductance recovery to its 2 initial value. Fig. 4 shows the conductance response G rG of gas air both deposited and a screen-printed WO films sin- 3 tered at the optimum temperatures of 480 and 500ЊC, respectively, to 10 ppm of H S in synthetic air as a 2 function of the operation temperature. The maximum response appears at room temperature for both types of films. The response of the deposited WO film 3 decreases exponentially with increasing T .AtT ) oo 210ЊC, the ratio G rG of both deposited and gas air screen-printed films is only of the order of 10. Fig. 3. Conductance response G rG vs. sintering temperature for gas air deposited and screen-printed nanocrystalline WO films exposed to 3 10 ppm of H S in synthetic air at room temperature. 2 Fig. 4. Conductance response G rG vs. operation temperature gas air for deposited and a screen-printed nanocrystalline WO films sin- 3 tered at 480 and 500ЊC, respectively, exposed to 10 ppm of H S in 2 synthetic air. Fig. 5 shows results of a more detailed study on the time dependence of room-temperature conductance, Ž. GtrG , of a deposited WO film sintered at 480ЊC air 3 and a screen-printed film sintered at 500ЊC during repeated exposures to increasing concentrations of H S 2 in synthetic air. Heating for a short time up to 250ЊC followed each exposure of the films to H S. It is 2 evident that the films were able to detect 1 ppm of H S 2 in synthetic air at room temperature during the time span of 10 min or more. Furthermore, it is found that the nanocrystalline WO films recovered their initial 3 conductance after the short annealing treatment at Ž. 250ЊC Fig. 5 irrespective of the H S exposure. 2 The nanocrystalline WO films ᎏ both deposited 3 and screen-printed ᎏ were not sensitive at room tem- perature to other tested gases, such as 100 ppm of CO, 10 ppm of NO, 500 ppm of H , 100 ppm of SO and 10 22 ppm NO in synthetic air. Only the conductance of the 2 screen-printed films sintered at 500ЊC 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 Our earlier X-ray diffraction studies of deposited WO films in as-deposited form and after sintering at 3 different temperatures up to 600ЊC showed a gradual phase transition to take place at T ) 400ЊC with a s tetragonal phase changing to a monoclinic phase wx together with an increase of the grain size 24 . The tetragonal phase was practically absent in the films sintered at T ) 600ЊC and the grain size of the mono- s clinic phase increased to 78 nm for T s 700ЊC. s The conductance response to H S of the deposited 2 films at room temperature was found to have a maxi- mum after sintering at 480ЊC. Clearly, an ‘activation’ () J.L. Solis et al. r Thin Solid Films 391 2001 255᎐260 259 Ž. Ž. Fig. 5. Conductance response vs. time, GtrG , a of a deposited air Ž. WO film sintered at 480ЊC and b of a screen-printed WO film 3 3 sintered at 500ЊC at repeated exposures to different concentrations of H S in synthetic air at room temperature. A temperature pulse up 2 to 250ЊC follows each H S exposure. 2 process was inherent in the sintering procedure. We note that a similar ‘activation’ to H S was found in 2 wx earlier work 25 on gold doped WO films deposited 3 by RF sputtering upon annealing them at 400ЊC. Our X-ray studies of the structure of the deposited WO films sintered at 480ЊC showed that they consist 3 of a mixture of tetragonal and monoclinic phases and the same is true for the screen-printed films sintered at 500ЊC. The sintering produces contacts between grains, many of which are between grains having different crystal structure. These grain contacts contribute sig- nificantly to the electrical conduction and presumably to the gas-sensing properties of the nanocrystalline WO films. 3 The conductance response to H S at room tempera- 2 ture was found to almost disappear after sintering of the screen-printed WO films above 600ЊC. Therefore 3 the room temperature H S sensitivity may be related 2 to the presence of grains having a tetragonal phase in wx the films. We may note here that Tamaki et al. 9 found that the gas sensitivity was controlled by the grain-size effect in WO films with grains smaller than 3 33 nm. The oxygen ions adsorbed at low temperatures on oxide semiconductor surfaces are thought to be O y , 2 wx even at room temperature 26 . Consequently, the elec- tron 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 wx work by Fang et al. 17 , the H S response was related 2 to a catalytic reaction of H S with the adsorbed O y 22 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 wx model of Wang et al. 27 , a small grain size, such as in the deposited nanocrystalline WO films after sintering 3 at 480ЊC, improves the gas sensitivity. 6. Conclusions We prepared nanocrystalline WO films with unique 3 and excellent sensing properties upon exposure to low concentrations of H S in air at room temperature. A 2 tetragonal phase present in the WO films may be 3 responsible for their high room-temperature H S sen- 2 sitivity. The optimum sintering temperature for H S 2 sensing was found to be approximately 480ЊC and 500ЊC, respectively, for deposited and screen-printed WO 3 films. In addition to having a high H S sensitivity, the 2 films displayed a stable performance after short heating pulses at 250ЊC. The pulses were necessary to speed up the conductance recovery after H S exposure. The 2 nanocrystalline WO films were able to detect 1 ppm of 3 H S in synthetic air at room temperature and did not 2 have any noticeable cross sensitivity to CO, NO, H , 2 SO and NO . 22 Acknowledgements This work was supported by Swedish Foundation for Strategic Research through its program on Advanced Ž Micro Engineering and the Academy of Finland pro- . jects ࠻37778 and ࠻44588 . References wx Ž. 1 H. Gleiter, Prog. Mater. Sci. 33 1989 223. wx Ž. 2 H. Gleiter, Mater. Sci. 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