influence of binders on the sensing and electrical characteristics of wo3-based gas sensors

The electron concentration of WO film around room temperature and the3 3 temperature dependence of film resistance in normal air do not show any systematic dependence on the employed bin

Influence of binders on the sensing and electrical characteristics of

Jong-In Yang a, H Lim a,), Sang-Do Han b a

Department of Electronic Engineering, Ajou UniÕersity, Suwon 442-749, South Korea

b

Korea Institute of Energy Research, Taejon 305-600, South Korea

Received 27 April 1999; received in revised form 3 May 1999; accepted 7 May 1999

Abstract

Ž

The effects of various binders added in the solidification process, such as polyvinyl alcohol PVA , silica sol and Al O , on the2 3 sensing and electrical characteristics of WO -based n-type semiconductor gas sensors are investigated Grain sizes show a slight variation3 according to the employed binders In the case of WO films fabricated with silica sol or Al O binder, some residue of binders at grain3 2 3 boundary and agglomerates of WO grains are observed The electron concentration of WO film around room temperature and the3 3 temperature dependence of film resistance in normal air do not show any systematic dependence on the employed binders In NO gas,x however, the optimum operation temperature and the sensitivity of WO sensors at that temperature are observed to depend greatly on the3 employed binders The resistance of the WO films shows an exponential temperature dependence in NO gas in the temperature range of3 x 110–3758C, and the increase of film resistance in NO gas is observed to depend greatly on the binders added in WO films Sensitivityx 3

to various ambient gases does not show any systematic variation All these results mean that the binders affect the sensing characteristics

of WO -based gas sensors mainly through the modification of chemical conditions at grain boundary rather than the modification of grain3 size and electron concentration q 1999 Elsevier Science S.A All rights reserved.

Keywords: WO NO gas sensor; Grain boundary barrier height; Binder effects3 x

1 Introduction

The semiconductor gas sensing devices are based on the

conductivity change of the semiconductor material due to

its interaction with gas When gas molecules are adsorbed

on the surface of a semiconductor, electron transfer occurs

between the semiconductor and the adsorbates If the

electron affinity of the adsorbates is larger than the work

function of the n-type semiconductor, the adsorbates

ac-cept electrons from the semiconductor In the opposite

case, the semiconductor accepts electrons from the

adsor-bates This electron transfer continues until the Fermi level

of the gas-adsorbed semiconductor surface becomes equal

to that of the bulk As a result of this electron transfer, a

depletion or accumulation of charges occurs near the

semi-conductor surface Then the accompanying variation of

surface potential barrier induces a change in the electrical

w x

conductivity or resistivity 1 Therefore, under the

ambi-)

Corresponding author

ence of oxidizing gas such as NO , electrical resistivity ofx

a polycrystalline n-type semiconductor film is increased due to the increase of the potential barrier height at the

surface andror grain boundary GB of the polycrystal In the ambience of reducing gas such as CO and H , electri-2 cal resistivity decreases

WO is an n-type semiconductor whose electron con-3 centration is determined mainly by the concentration of stoichiometric defects such as oxygen vacancy like any other metal oxide semiconductors The first work on the feasibility of WO films as a gas sensor was reported by3

w x

Shaver 2 , who observed that the conductivity of WO3 films changed greatly upon the exposure to the H2 ambi-ent Following this pioneering work, many works have been performed on the structural and electrical properties and sensing characteristics of WO films made by various3

w x

methods such as sol–gel coating 3 , magnetron sputtering

w x4 , thermal evaporation 5 , chemical vapor deposition 6 w x w x

Recently, WO -based gas sensor has exhibited an excellent3 sensing characteristic for NO gas irrespective of the WOx 3 0925-4005r99r$ - see front matter q 1999 Elsevier Science S.A All rights reserved.

PII: S 0 9 2 5 - 4 0 0 5 9 9 0 0 2 4 8 - 8

w x

structure whether it is amorphous or polycrystalline 3–8

It has also demonstrated a good selective detectability for

the NOx gas with an excellent cross-sensitivity for the

w x

interference gases such as CO and CH4 5 In general, the

sensing characteristics, such as sensitivity and selectivity,

of the polycrystalline sensor depend greatly on the

mi-crostructural properties of the polycrystal and the metal

impurities doped as catalysts 8,9 In the case of sol–gel

coated WO3 films, the binders are commonly added to

enhance the solidification The binders also influence the

sensor characteristics 3,10 , since the microstructure of

w x

sol–gel coated WO can be affected by the binders 3 For3

example, the grain size of WO3 film decreases as the

w x

amount of SiO binder is increased 3 Thus the addition2

of a suitable amount of SiO in WO sol–gel is believed2 3

to increase the porosity of the composite film and in turn

w x

the sensitivity of the sensor 3 To our knowledge,

how-ever, the origin of the binder effects on the sensing

charac-teristics of WO films has not been clarified, since most of3

the studies are concerned with the sensing characteristics

related with fabrication methodology andror

microstruc-ture of the polycrystals

In this paper, we have investigated the effects of binders

on the sensing characteristics and the structural–electrical

properties of WO films The physical properties of WO3 3

films, such as grain size and the electron concentration

around room temperature, show little dependence on the

employed binders But the sensing characteristics and the

electrical resistance in NOx gas depend greatly on the

binders It is concluded, from these facts, that the property

mostly affected by the binders is GB barrier heights in the

ambience of gas and this phenomenon is related to the

chemical nature at GB It is also discussed that optimum

operation temperature of the sensors for NO gas is simplyx

determined by the ratio of electrical resistances in NO gasx

to that in normal air rather than the temperature

depen-dence of adsorptionrdesorption kinetics of NO gas.x

2 Grain boundary potential barrier and sensitivity

The operation of a semiconductor gas sensor is realized

through the change of the surface andror interface

poten-tial barrier due to the adsorbed radicals Although an actual

WO3 polycrystal has a random GB potential barrier

net-work, suppose that it has a planar type interface for the

simplicity of discussion As a result of gas adsorption at

GB, electrons are depleted from the GB region, and the

potential barrier height V at GB is given byi

qN Wd 2

2 e

under the depletion approximation Here, q is the

elec-tronic charge, N the donor concentration, W the depletion

region width, and e is the permittivity of the semiconduc-tor From the charge neutrality condition between the

interface charge Q and the space charge in depletioni

regions, we obtain

In actual polycrystalline films with rather small grain size like ours, the extent of the space-charge layer depends

on the Debye length given by

1r2

e kT

and the contacting shape of GB between grains 1,11

Here, k, T, and n are the Boltzman constant, absolute

temperature, and the electron concentration in the grain, respectively Even in this case, the charge neutrality condi-tion should always be satisfied, and the electrons are filled

up to the so-called neutral surface Fermi level 12

Ac-3

Ž

cording to Tersoff, the sp hybrid energy the dangling

bond energy plays the role of neutrality level for the

intimate contact between covalent semiconductors 13 The interface formed on an etched semiconductor surface andror on the GB of a polycrystal is not an intimate contact, and the neutral surface Fermi level of this non-in-timate contact varies according to the species and the

amount of adsorbed atoms 14 This fact means that the potential barrier height of the GB of a given material may depend on the material processing or the chemicals used It also indicates that the gas molecules adsorbed at the GB

can change the interface charge Qi and the potential

barrier height V by changing the distribution of interfacei

states and the electron occupation on them In some cases, even the concentration of the adsorbed gas ions can be

measured from the change of potential barrier 15 The potential barrier at the GB of the polycrystal im-pedes the flow of the electrons between the grain Then, due to the potential barrier at GB, the conductance of a

w x

polycrystal has an activation form as 1

yqVi

where G is the conductance of the grain itself When ano

oxidizing gas like NO is diffused into the GBs of WOx 3

film, the absolute value of interface charge Q increasesi and thus the GB barrier height changes from V to V q DV i i i

Therefore, the resistance between two electrodes in NOx gas is given by

q V q DVŽ i i.

kT

where R is the resistance when the GB potential barrier iso absent Therefore, when the sensitivity S is defined as the

ratio of the resistance in NO gas to that in normal air, S is x

given by

Thus, the influence of binders on the sensitivity of the

WO3 gas sensors can be studied by observing DV in ai

given gas ambient for each of the employed binders

3 Experiments

In this study, WO3 powder mixed with 4 wt.% TiO2

were prepared by the sol precipitation method from the

appropriate mixture of TiCl and WCl The film with this4 6

composition has been observed to show the best sensitivity

to NOx gas in various WO3 films doped with different

species and amount of catalysts 10,16 The powder mixed

with methanol was then ball-milled for 30 min without

binder or with the binder of polyvinyl alcohol PVA ,

silica sol, and Al O , respectively The concentration of2 3

the added binders was 5 or 10 wt.% Finally the sensor

material paste was screen-printed on the alumina substrate

and then annealed at 8008C for 2 h in an air flow

condi-tion For the electrodes of the sensor, interdigitized Au

contacts were formed on the front surface of the alumina

substrate before the paste printing Pt was also deposited

on the back surface of the substrate as a heating resistor

for the sensor Fig 1a shows the cross-sectional view of

the sensor used in this work

The microstructural analysis of the samples was

per-formed using a Philips 515 scanning electron microscope

Fig 1 a Cross-sectional view of the employed film gas sensors and b

the measurement circuit.

ŽSEM and an H-7100 transmission electron microscope

ŽTEM Source NO gas was obtained by mixing NO gas x

and NO2 gas with the volume ratio of nine to one The desired NO gas ambience was provided by injecting thex prescribed amount of NOx gas to a chamber with the dimension of 40 = 25 = 18 cm3 and then stirring the gas mixture with a small fan for 3 s The sensing character-istics of the samples were measured in this closed chamber using the circuit shown in Fig 1b The sample resistance

was measured with the control voltage of V s 10 V, andC the sensitivity S of the sample was defined as the ratio of the resistance in gas to that in normal air, RgasrR air

Sensor temperature was controlled by varying the heater

voltage VH in Fig 1b and was measured using a Minolta

IR 0506C spot thermometer The temperature dependence

of the sample resistance, in NO gas or in normal air, wasx measured by an HP 4194A RLC meter with the probing signal of 100 kHz and 20 mV Before the measurement of the temperature-dependent sample resistance in NO gas,x the samples were maintained long enough time in NOx ambience at the predetermined optimum operation

ature see Fig 3 for the stabilization of gas adsorption Then the resistance was measured raising the sample tem-perature after the samples had been cooled down to room temperature

4 Results and discussion

From the Hall measurements performed in normal air, the room-temperature electron concentration of all the samples was found to be about 1 = 1017 cmy 3

with a slight sample-to-sample variation However, we could not find any systematic influence of employed binders on the carrier concentration Electron concentration was observed

to increase slightly in the temperature range of 80–2308C after a rather fast increase between 30 and 808C Electron concentration is thus believed to be mainly determined by the concentration of oxygen vacancies like any other metal oxide semiconductors The rather fast increase of electron concentration around 808C observed in some samples might

be due to the electron emission from the interface traps andror some deep defects The average grain size, deter-mined by the TEM observation on the sintered samples, was 61 nm for the samples fabricated without binder and

56, 58, and 61 nm for the samples fabricated with 5 wt.% binders of PVA, silica sol and Al O , respectively The2 3 SEM observation revealed that the grain sizes were dis-tributed more homogeneously in the samples fabricated without binder or with a PVA binder compared with those fabricated with silica sol or Al O2 3 binder We also ob-served from the energy dispersive X-ray spectroscopy that about 3% of SiO2 and 3.5% of Al O2 3 remained at the grain edges of the samples fabricated with silica sol and

Al O binders, respectively We could not find any PVA

residue in the samples fabricated with PVA binder which

had the smallest grain size The added PVA is thus

be-lieved to have almost completely evaporated in the

sinter-ing process at 8008C

Typical sensing characteristics are shown in Fig 2 for

the samples fabricated without binder and with 5 wt.%

PVA binder One can note that the sensor fabricated with

PVA binder has better response to NO gas in sensitivityx

and response time compared with the one fabricated

with-out binder One can also notice that the WO film resis-3

tance is decreased in CO gas since it provides a reducing

gas ambience However, the response of WO films to CO3

Ž

gas is very small compared with that to NO gas Note thex

differences in ambient gas concentration and output

volt-

ages It is therefore clear that these sensors, especially the

one fabricated with PVA binder, can selectively detect

NO gas from the mixture of NO and CO.x x

Fig 3 shows the temperature dependence of the

sensi-tivity in 15 ppm NO gas for the samples that are differentx

in species and amount of binders As can be seen in this

figure, the sensitivity S and the optimum operation

temper-ature T depend on the binders used For the employedo

binders, the samples fabricated with 5 wt.% binders show

Ž

better sensing characteristics i.e., lower T and higher So

value compared with the samples with 10 wt.% binders

except for the case of the sensitivity of the silica sol added

film The sample fabricated with 5 wt.% PVA binder

shows best sensing characteristics, i.e., it shows highest S

value at lowest To S s 375 at T s 1808C But addingo

silica sol or Al O2 3 binder reduces the sensitivity of

WO :TiO film to NO gas Table 1 shows the response3 2 x

time and the recovery time of the samples in the same NOx

gas concentration One can note that the samples fabricated

with binders respond faster to the NO gas than the onex

Fig 2 Typical output voltage responses of the sensors at various

concen-trations of NO and CO gases.

Fig 3 Operation-temperature dependence of the sensitivity in 15 ppm

NO gas for the samples fabricated with different species and amount ofx binders The WO :TiO3 2 film with 5 wt.% PVA binder shows best sensing characteristics.

fabricated without binder But the samples fabricated with PVA binder, which show highest sensitivity, recover most slowly in the normal air

Hereafter, we consider the WO films fabricated with 53 wt.% binders only, since they show better sensing charac-teristics than the ones fabricated with 10 wt.% binders The dependence of sensitivity on NOx concentration is shown in Fig 4 As can be seen in this figure, the sensor fabricated with the PVA binder shows the highest sensitiv-ity consistently for all the employed NO concentration.x One can also note that the sensitivity of the samples fabricated with binders increases steeply up to 30 ppm of

NOx and then increases much slowly, while that of the sample without binder increases rather steadily Sensitivity

is related to the variation of GB barrier height according to

Ž

Eq 6 Therefore, the saturation of sensitivity around 30 ppm of NO , observed especially in the samples withx binders, means that the GB barrier height does not increase further above this NO concentration presumably due tox the consumption of available sites at GB for the NOx molecule adsorption In this context, a slow increase of the

Table 1 Response and recovery times of the studied sensors in 30 ppm NO gas atx

each To

Binders wt.% None PVA Silica sol Al O2 3

Ž

Ž

Fig 4 Dependence of sensitivity of the WO :TiO 3 2 films with different

binders on the NO concentration at optimum operation temperatures.x

sensitivity of the samples with SiO2 binder and without

binder for the NOx concentration higher than 30 ppm

might be due to a further penetration of NOx gas deep

from GB under the ambience of high NO concentration.x

As described in Section 2, sensitivity is mostly related

to the difference in the effective GB potential barrier in

gas and in normal air due to the adsorptionrdesorption of

gas molecules at GB interface The effective potential

barrier height hindering electron transport is affected by

w x

the grain size and the Debye length 9 In our case, the

electron concentration does not show a large

sample-to-sample dependence Thus the influence of Debye length on

the binder-dependent sensing characteristics should be

marginal One may imagine that the grain size affects

some sensing characteristics of our samples since the NOx

sensitivity of WO film with small grain size can be larger3

w x

than that with large grain size 8 For example, the

samples fabricated with PVA binder show highest

sensitiv-ity and longest recovery time Since these samples have

the smallest grain size in our samples, the ratio of surface

area to volume should be largest Moreover, small pores

would be created as the PVA binder evaporates in the

annealing process at 8008C Thus these samples may show

a higher sensitivity compared with any other samples NOx

gas would be desorbed in normal air rather slowly after

being adsorbed at the GB andror small pores that lie deep

from the sample surface A similar reasoning might also

explain why the sample with PVA binder responds to the

NOx gas more slowly compared with the samples with

silica sol and Al O binders However, the dependences of2 3

optimum operation temperature and GB barrier heights in

NO gas on the employed binders Figs 3 and 5 cannotx

be explained by the grain size effects

The influence of binders on the sensor characteristics is investigated more systematically by directly measuring the temperature dependence of sensor resistance as described

in Section 3 Fig 5 shows the temperature dependence of the samples fabricated without binder or with binders in normal air with 50% relative humidity and in 30 ppm NOx gas, respectively As can be seen in this figure, all the samples show similar temperature dependence in normal air, and the film resistance shows an exponential tempera-ture dependence only in a narrow temperatempera-ture range The samples have a rather high electron concentration of the order of 1017 cmy 3 Electrons can then easily tunnel through the small and narrow GB barriers Thus the scat-tering mechanism other than GB scatscat-tering can be impor-tant especially at high temperatures The fast variation of electron concentration at about 758C is believed to affect the variation of resistance in this temperature range Be-cause of these phenomena, the film resistance would show

an exponential temperature dependence only in a narrow temperature range The GB barrier height in normal air is estimated to be about 0.17–0.25 eV from the resistance values in the temperature range of 100–2008C In 30 ppm

NO gas, the sample resistances increase by more than onex order of magnitude They also show exponential tempera-ture dependences in a wider temperatempera-ture range above 1108C These facts clearly indicate that the GB barrier heights are increased due to the adsorption of oxidizing

NO gas, i.e., the anionic adsorption of NO molecules,x x

Fig 5 Variation of resistance as the sample temperature is varied in normal air condition and in the ambience of 30 ppm NO In the case ofx normal air, the relative humidity was 50% at room temperature The film resistance is increased and shows an exponential temperature dependence above 1108C in 30 ppm NO gas.

and maintained up to a rather higher temperature The

non-exponential temperature dependence below 1108C in

NO gas might also be related to a rather fast variation ofx

electron concentration in this temperature range If the

decrease of sensitivity above the optimum operation

perature To see Fig 2 is due to the desorption of NOx

gas, then the resistance should decrease with the increase

of temperature faster than the exponential one since the

desorption effect adds on the exponential temperature

de-Ž

pendence given by Eq 5 But the decrease of film

resistance is faster than the exponential one only above

3758C in some samples Therefore, the decrease of

sensi-tivity in the temperature range of 200–3508C observed in

Fig 3 is not due to the desorption of NOx gas The

decrease of sensitivity due to the desorption of NO gas, ifx

it exists, should appear only above 3758C where the

resis-tance decreases faster than the exponential one in some

samples

One can also notice in Fig 5 that the increase of

resistance in NOx gas is different for the samples

fabri-cated with different binders For example, the samples

with PVA silica sol binder that shows the smallest

Žlargest resistance values in normal air show largest

Žsmallest resistance in NO gas The values of GB poten- x

tial barriers deduced from the exponential temperature

dependence of resistance in the temperature range of 110–

3758C are given in Table 2 for the ambience of 30 ppm

and 120 ppm NO gases One can see that the GB barrierx

heights in NO gas depends quite on the choice of binders.x

As mentioned earlier, the grain size of WO :TiO3 2

poly-crystals has little dependence on the choice of binders

Moreover, Debye length is comparable in all samples

Thus the origin of the observed difference in GB barrier

heights in NO gas should be due to the dependence ofx

anionic adsorption at GB on the employed binders

An-ionic adsorption can occur differently due to the different

chemical compositions at GB when different binders are

employed This is quite possible since some binders can

remain at GB as observed in the samples fabricated with

silica sol or Al O binder, and then the different choice of2 3

binders would result in the different oxygen deficiency at

GB For example, the sample with the PVA binder may

have a larger oxygen deficiency at GB compared with the

samples with Al O or SiO binder and thus can provide2 3 2

more sites for the NO adsorption Since the compositionx

at GB depends on the binders, the distribution of interface

states that affect the GB barrier height would do so, too

We can thus safely say that the binders influence the

Table 2

Typical values of GB potential barriers in meV in the ambience of 30

and 120 ppm NO gasesx

Fig 6 Histogram for the sensitivities of the WO :TiO3 2 films with different binders in various ambient gas conditions The sensitivity scale

is shown in left and right axes for NO gas and other gases, respectively.x

Sensors fabricated with binders show better selectivity to NO gas.x

sensing characteristics of the polycrystalline WO sensors3 mainly through the chemical nature of GB and the distribu-tion of interface states

Fig 6 shows the variation of sensitivity according to the various binders and ambient gases employed For the gases other than NO , a high gas concentration had to be main-x tained to see a slight difference in sensitivity Each sensi-tivity value is measured at the relevant optimum operation temperature for each sample and ambient gas Noting the difference in the sensitivity scales for NO gas and others,x

we can see that the WO films, especially the ones fabri-3 cated with binders, show a really excellent selectivity to

NO gas One can also notice that the sensors do not showx any systematic trend in sensitivity except that the sensors fabricated with binders have a lower sensitivity than the one fabricated without binder for the ambient gases except

NO If the binders affect the sensitivity through thex physical properties of the polycrystal, such as grain size andror electron concentration of the grain, the sensitivity

is expected to show a general trend to the binders, since the above physical properties would systematically affect the electrical resistance Thus the results in Fig 6 is another evidence showing that the binders affect the sens-ing characteristics of WO film mostly through the chemi-3 cal nature of GB rather than the modification of physical properties of polycrystals

5 Conclusion

The origin of binder effects on the sensing character-istics of WO3 gas sensors is investigated for the PVA, silica sol, and Al O binders The binders are observed to

influence little the physical properties of sintered WO3

films such as grain size and electron concentration around

room temperature They are, however, observed to affect

greatly the sensing characteristics of films for various

ambient gas and the electrical resistance in NO gas It isx

discussed, from these facts, that the binders affect the gas

sensing characteristics of WO film through the modifica-3

tion of the chemical nature of the GB rather than the

modification of physical properties of the polycrystal It is

also discussed that the optimum operation temperature of

WO3 gas sensor to NOx gas is not determined by the

adsorptionrdesorption kinetics of NOx gas but by the

simple temperature dependence of RgasrRair ratio

Acknowledgements

Parts of this work was supported by the BSRI program

Ž98-015-D00056 of the Korean Ministry of Education

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Jong-In Yang received the B.E degree from Kangnung National Univer-sity and M.E degree from Ajou UniverUniver-sity in electronic engineering in

1997 and 1999, respectively He is now working as a development engineer in Korea LPE Products His current research activities are on the development of III–V LEDs and sensors.Han-jo Lim was born in Kyung-buk, Korea in 1947 and obtained M.S degree in physics from Seoul National University, Korea in 1974 He received the PhD degree in solid state physics from the University of Montpellier II, France in 1982 He was appointed as an assistant professor in the electronic engineering department of Ajou University, Korea in the same year His research field includes semiconductor physics, semiconductor devices and material characterization, and electron device reliability He has published about

80 papers on the international journals and 30 papers in the domestic journals.Sang-Do Han received his PhD degree in solid state chemistry, University of Bordeaux, France in 1994 He worked at LG semiconductor for 1978–1980, and is currently working at Korea Institute of Energy Research since 1980 His main research interests are the electronic materials, chemical sensors and their applications He serves as the editor

of Journal of the Korean Sensors Society.