semiconductor gas sensors based on nanostructured tungsten oxide

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

Semiconductor gas sensors based on nanostructured tungsten

oxide

J.L Solisa,1, S Saukkob, L Kisha,2, C.G Granqvista, V Lanttob,U

aDepartment 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

gas evaporation was used in both cases either for a direct deposition of films deposited films or to produce ultra fine WO3 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 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 s480⬚C and at T s500⬚C, respectively 䊚 2001 Elsevier Science B.V All rights s

reserved.

Keywords: Tungsten oxide; Gas deposition; Nanocrystalline; H S sensing; Gas sensor2

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 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

w x form a conductance change 5,6 A discussion of the

role of the size and shape of the contacts necks

UCorresponding author: Tel.: 5532712; fax:

q358-8-5532728.

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

w x

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

rial 1᎐4 Gas sensing applications of nanocrystalline materials have received considerable interest in recent

w x years 7 and it is well known that the gas sensitivity of

both porous SnO2 8 and WO3 9 films increases with 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

w x conductance-based gas sensing 10 There are only a few earlier studies of the gas sensing properties of nanocrystalline WO3 films In those cases the films were prepared by evaporation of

w x tungsten in the presence of O2 11 , or by the sol᎐gel

w x method 12 In the present work we employ advanced

reactive gas evaporation 13,14 both to deposit directly

nanocrystalline WO3 films deposited films and to 0040-6090 r01r$ - see front matter 䊚 2001 Elsevier Science B.V All rights reserved.

PII: S 0 0 4 0 - 6 0 9 0 0 1 0 0 9 9 1 - 9

Ž produce ultra-fine particles UFP of WO for screen3

printing of WO thick films screen-printed films Gas3

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

Žtypically in the range 200 to 500⬚C , which implies that

power is required In recent years there has been a

w x 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

w x16 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

ported that ZrO2᎐SnO 17 and ZnO 18 sensors can2

be used to detect H S and NH at room temperature,2 3

respectively, but their sensitivity was low

We report in this work that both deposited and

screen-printed nanocrystalline WO3 films sintered at

480⬚C and at 500⬚C, respectively, exhibit excellent

sens-ing properties for small quantities of H S in air even at2

room temperature Structural studies gave some

sup-port to the tetragonal WO phase, that is stabilized in3

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

w x 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=10y2 mbar

thetic air 80 vol.% N2 and 20 vol.% O2 was then

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

w x size distribution is narrow 20 The starting material

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

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-direc-tions by a digital programmable controller; the scan-ning speed of the substrate was 1.5 mm sy1 The alumina substrates had preprinted gold electrodes be-ing 0.2 mm apart and a Pt heatbe-ing resistor printed on

the reverse side Rectangular 3=2.5 mm nanocrys-talline WO3 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

tempera-tures T in the 200 s -T -600⬚C range for 1 h s

2.2 Screen-printed films

The ceramic UFP mode of the advanced gas evap-oration unit referred to above was used to produce nanocrystalline WO3 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 N gas stream of 5 l min2 y1 and an atmo-spheric 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 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 WO3 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 WO3 nanopowder After mixing the powder with the vehicle, the paste was milled in a triple mill in order to homog-enize the mixture The WO3 thick films were then

Ž screen printed on alumina substrates Rectangular 3=

2 2.5 mm nanocrystalline WO films with a thickness of3 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-T -800⬚C range s

2.3 Measurements

The crystal structure and crystallite size of the nanocrystalline WO films were determined by X-ray

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

ex-amined by a field-emission scanning electron

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

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 o in the 20- T - 300⬚C range with a o

computer-controlled measuring system employing the

w x flow-through principle 23 H S, H , CO, NO, NO2 2 2

and SO at various concentrations in dry synthetic air2

were used as test gases in the measurements

3 Structural properties

The as-deposited nanocrystalline WO3 films

pre-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-w x

rer’s formula 21 This size is in agreement with earlier

w x

results 11 for WO powder fabricated by gas evapora-3

tion The screen-printed WO3 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

w x temperature structure of WO , stable above 7703 ⬚C 22

Clearly, the high temperature associated with the WO3

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 films3

sintered at 480⬚C and 500⬚C, respectively Reflection

peaks belonging to monoclinic as well as tetragonal

phases of WO are marked in the figure The sharp3

peaks due to substrate are also indicated

Fig 2a shows a SEM micrograph of the morphology

of the deposited nanocrystalline WO film sintered at3

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 film sintered3

at 500⬚C 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 500 3 ⬚C, respec-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 WO3

Ž film sintered at 480 ⬚C and b FESEM micrograph from a screen-printed WO film sintered at 500 ⬚C.

GgasrG , where G and G denote the conductanceair gas air

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

sinter-ing temperature was found to play an important role

for the gas sensing properties Fig 3 shows the

conduc-tance response GgasrG to H S at room temperatureair 2

of both deposited and screen-printed nanocrystalline

WO3 films sintered at different temperatures in the

300-T -800⬚C range The films were subjected to 10 s

min of exposure to 10 ppm of H S in synthetic air at2

T o s20⬚C The conductance response G rG of thegas air

deposited films increases with increasing T up to 480 s ⬚C,

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 s s)600⬚C

The conductance of the deposited film sintered at

480⬚C increased by approximately three orders of

mag-nitude 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 approx 1 min at 250⬚C after the exposure

to H S yielded a rapid conductance recovery to its2

initial value

Fig 4 shows the conductance response GgasrG ofair

both deposited and a screen-printed WO3 films

sin-tered at the optimum temperatures of 480 and 500⬚C,

respectively, to 10 ppm of H S in synthetic air as a2

function of the operation temperature The maximum

response appears at room temperature for both types

of films The response of the deposited WO3 film

decreases exponentially with increasing T At T o o)

210⬚C, the ratio G rGgas air of both deposited and

screen-printed films is only of the order of 10

Fig 3 Conductance response GgasrG vs sintering temperature forair

deposited and screen-printed nanocrystalline WO films exposed to3

10 ppm of H S in synthetic air at room temperature.

Fig 4 Conductance response GgasrGair vs operation temperature for deposited and a screen-printed nanocrystalline WO3 films sin-tered at 480 and 500 ⬚C, 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

G t rG , of a deposited WO film sintered at 480⬚Cair 3

and a screen-printed film sintered at 500⬚C during repeated exposures to increasing concentrations of H S2

in synthetic air Heating for a short time up to 250⬚C followed each exposure of the films to H S It is2 evident that the films were able to detect 1 ppm of H S2

in synthetic air at room temperature during the time span of 10 min or more Furthermore, it is found that the nanocrystalline WO3 films recovered their initial conductance after the short annealing treatment at

250⬚C Fig 5 irrespective of the H S exposure.2

The nanocrystalline WO3 films ᎏ both deposited 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 102 2 ppm NO in synthetic air Only the conductance of the2 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

Our earlier X-ray diffraction studies of deposited

WO films in as-deposited form and after sintering at3 different temperatures up to 600⬚C showed a gradual

phase transition to take place at T s)400⬚C with a tetragonal phase changing to a monoclinic phase

w x together with an increase of the grain size 24 The tetragonal phase was practically absent in the films

sintered at T s)600⬚C and the grain size of the

mono-clinic phase increased to 78 nm for T ss700⬚C

The conductance response to H S of the deposited2 films at room temperature was found to have a maxi-mum after sintering at 480⬚C Clearly, an ‘activation’

Ž Ž

Fig 5 Conductance response vs time, G t rG , a of a depositedair

Ž

WO film sintered at 480 3 ⬚C and b of a screen-printed WO film 3

sintered at 500 ⬚C at repeated exposures to different concentrations

of H S in synthetic air at room temperature A temperature pulse up2

to 250 ⬚C follows each H S exposure 2

process was inherent in the sintering procedure We

note that a similar ‘activation’ to H S was found in2

w x

earlier work 25 on gold doped WO films deposited3

by RF sputtering upon annealing them at 400⬚C

Our X-ray studies of the structure of the deposited

WO films sintered at 4803 ⬚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

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 6003 ⬚C Therefore

the room temperature H S sensitivity may be related2

to the presence of grains having a tetragonal phase in

w x 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 than3

33 nm

The oxygen ions adsorbed at low temperatures on oxide semiconductor surfaces are thought to be Oy2,

w x 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

w x work by Fang et al 17 , the H S response was related2

to a catalytic reaction of H S with the adsorbed O2 y2 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

w x model of Wang et al 27 , a small grain size, such as in the deposited nanocrystalline WO films after sintering3

at 480⬚C, improves the gas sensitivity

6 Conclusions

We prepared nanocrystalline WO films with unique3 and excellent sensing properties upon exposure to low concentrations of H S in air at room temperature A2 tetragonal phase present in the WO3 films may be responsible for their high room-temperature H S sen-2 sitivity The optimum sintering temperature for H S2 sensing was found to be approximately 480⬚C and 500⬚C, respectively, for deposited and screen-printed WO3 films In addition to having a high H S sensitivity, the2 films displayed a stable performance after short heating pulses at 250⬚C The pulses were necessary to speed up the conductance recovery after H S exposure The2 nanocrystalline WO films were able to detect 1 ppm of3

H S in synthetic air at room temperature and did not2 have any noticeable cross sensitivity to CO, NO, H ,2

SO and NO 2 2

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噛37778 and 噛44588

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