gas sensing properties of nanoparticle indium - doped wo3 thick films

It was observed that the pure nanoparticle WO3 sensing layers began to react with nitrogen oxide at room temperature.. Keywords: Pure and indium-doped tungsten oxide thick film; WO3 gas

Gas sensing properties of nanoparticle indium-doped WO 3 thick films

V Khatkoa,∗, E Llobeta, X Vilanovaa, J Brezmesa, J Hubalekb, K Malyszb, X Correiga

aDepartment d’ Enginyeria Electronica, Campus Sescelades, Universitat Rovira i Virgili, 43007 Tarragona, Spain

bDepartment ofMicroelectronics, Technical University ofBrno, 60200 Brno, Czech Republic

Available online 11 August 2005

Abstract

The gas sensing properties of pure and indium-doped nanoparticle WO3thick films were studied Sensors were prepared using commercial

WO3nanopowders and powder mixtures with different concentrations of In (1.5, 3.0 and 5.0 wt.%) The gas sensing properties of the sensors

to nitrogen dioxide, carbon monoxide, ammonia and ethanol were investigated It was observed that the pure nanoparticle WO3 sensing layers began to react with nitrogen oxide at room temperature These sensors had maximum sensitivity to NO2at 100◦C The indium-doped

WO3-based sensors were selective to NO2and CO at 200 and 300◦C, respectively A mechanism for such behaviour is discussed

© 2005 Elsevier B.V All rights reserved

Keywords: Pure and indium-doped tungsten oxide thick film; WO3 gas sensor; Gas sensing properties

1 Introduction

Nowadays, WO3-based thick films are considered as one

of the most interesting materials for detecting nitrogen oxides

and other species such as NH3, CH4and CO[1–6]

Reduc-ing the grain size of active layers is one of the key factors to

enhance the gas sensing properties of semiconductor metal

oxide sensors [7,8] Additionally, the inclusion of

differ-ent doping metals in the sensing films has been shown to

increase their sensitivity to specific gases In very recent

papers, the effects of doping either with In or indium oxide

on the response of tin oxide to hydrogen[9,10], methanol and

carbon monoxide[11]have been investigated An important

reduction in the operating temperature of the sensors for

opti-mal sensitivity to these species has been reported The aim

of this work is to study the sensing properties of nanoparticle

WO3gas sensors doped with In A mechanism of response

to NO2and CO will be presented and discussed

2 Experimental

Sensors were fabricated by screen-printing onto alumina

substrates A heating element and a temperature sensitive

∗Corresponding author Tel.: +34 977558653; fax: +34 977559605.

E-mail address: vkhatko@etse.urv.es (V Khatko).

meander on the backside substrate and interdigited gold elec-trodes on the front side of substrate were prepared by using commercial platinum (Heraeus C3657) and conductive (ESL 8884) pastes, according to the sensor fabrication procedures reported in[12] The commercial WO3nanopowder (Aldrich 55,008-6) with calculated spherical diameter up to 33.1 nm was mixed with InCl3using special technology to ensure: the breaking of In Cl bonds, the removal of Cl atoms and the uniform distribution of indium atoms in the tungsten oxide powder mixture Three powder mixtures with an In/W atom ratio equal to 1.5, 3.0 and 5.0 wt.% were prepared On the basis of the pure nanopowder and these powder mixtures, the gas sensing layers were deposited onto the electrodes using an organic binder Glass frit (4.9 wt.%) was used for the preparation of the sensing layers based on pure WO3 nanopowder, to ensure a good adhesion to the substrate After being deposited, the active layers were dried at 125◦C for

10 min and fired at 600◦C for 20 min.

The gas sensing properties of the sensors to nitrogen dioxide, carbon monoxide, ammonia and ethanol were inves-tigated The sensors were kept in a temperature and moisture controlled test chamber (40±1◦C and 41–43% R.H.) The

sensors were operated at ambient temperature, 100, 150, 200,

250, 300 and 350◦C to analyse the effect of the working

tem-perature on their response The resistance of the sensors in

the presence of either pure air (Rair) or the different pollutants

0925-4005/$ – see front matter © 2005 Elsevier B.V All rights reserved.

doi:10.1016/j.snb.2005.06.060

measurements using a Siemens D5000 diffractometer

oper-ated at 40 kV and 30 mA with Cu K␣ radiation

3 Results and discussion

3.1 Gas sensitivity studies

For the first set of experiments, the sensing layers based

on pure tungsten oxide nanopowders were used The WO3

sensors were operated in a working temperature range from

room temperature to 350◦C The gas sensing properties of

the sensors to nitrogen dioxide and carbon monoxide were

investigated WO3sensors did not change resistance

signifi-cantly during their exposure to increasing CO concentrations

up to 500 ppm, throughout the temperature range studied

The sensor response of these sensors to nitrogen dioxide had

definite features.Fig 1shows the influence of the presence

of glass frit on the response of un-doped tungsten oxide

sen-sors to nitrogen dioxide It can be seen that the sensen-sors began

to react with gas at room temperature (Fig 1a) At 1 ppm

of NO2 the responsiveness calculated using the expression

S = (Rgas− Rair)/Rairis shown inTable 1as a function of the

sensor working temperature In the temperature range from

room temperature to 150◦C the sensors with glass frit (WO2

3) show a higher response to nitrogen dioxide than the sensors

without glass frit (WO13) (see Fig 1b and c) Furthermore,

the response of the former is much faster than the response

of the latter At working temperatures higher than 150◦C the

sensor resistance increases monotonously (Fig 1d) and the

sensitivity to nitrogen dioxide decreases

In a second set of experiments, the level of In doping

in the response of tungsten oxide based sensors to nitrogen

dioxide and carbon monoxide was investigated The sensors

were operated in a temperature range from 150 to 350◦C.

Figs 2 and 3show the effects of indium concentration in the

detection of nitrogen dioxide and carbon monoxide,

respec-Table 1

Responsiveness of pure nanopowder WO3 sensing layers to NO2 (1 ppm) as

a function of the working temperature and thick film composition

Thick film composition Working temperature

Room temperature 100 ( ◦C) 150 (◦C)

Super-indexes 1 and 2 are for tungsten oxide films without and with glass

frit, respectively.

Fig 1 Responses to nitrogen dioxide of pure WO3 sensors without (WO1) and with (WO2) glass frit in the paste, operated at room temperature (a),

100 ◦C (b), 150◦C (c) and 200◦C (d).

Fig 2 Responses to nitrogen dioxide of indium-doped WO3 sensors with

indium concentrations of 1.5 wt.% (WIn1), 3.0 wt.% (WIn2) and 5.0 wt.%

(WIn3), operated at 150 ◦C (a) and 200◦C (b).

tively The In-doped WO3 sensors were more sensitive to

NO2 when operated at 200◦C and more sensitive to CO

when operated at 300◦C The sensors showed the highest

responsiveness to NO2when indium concentration was set at

3.0 wt.% When operated at 200◦C in the presence of 1 ppm

of NO2, the responsiveness of the In-doped tungsten oxide

sensors was 0.144, 0.347 and 0.188 for indium

concentra-tions of 1.5 wt.% (sensor labelled WIn1), 3.0 wt.% (WIn2)

and 5.0 wt.% (WIn3), respectively Fig 3 shows that only

the sensors (WIn3), i.e., doped with In at a level of 5 wt.%

responded to CO This responsiveness was up to 0.137

In a third set of experiments the response of the

differ-ent sensors to ethanol and ammonia was investigated The

working temperature range varied from 100 to 300◦C The

sensors showed response to ethanol at temperatures higher

than 200◦C.Fig 4shows the response sensor to ethanol as a

function of the concentration of indium in the sensing layers,

when operated at 300◦C It can be seen that a saturation effect

in sensor response takes place at 10 ppm of ethanol

Subse-quent injections of ethanol to increase its concentration to 100

Fig 3 Responses to carbon monoxide of indium-doped WO3 sensors with indium concentrations of 1.5 wt.% (WIn1 ), 3.0 wt.% (WIn2) and 5.0 wt.% (WIn3), operated at 250 ◦C (a) and 300◦C (b).

Fig 4 Responses to ethanol of indium-doped WO3 sensors with indium concentrations of 1.5 wt.% (WIn1 ), 3.0 wt.% (WIn2 ) and 5.0 wt.% (WIn3), operated at 300 ◦C.

WIn1 , WIn2 and WIn3 correspond to indium doping levels of 1.5, 3 and

5 wt.%, respectively.

and 500 ppm did not essentially change the resistance of the

sensing layers The responsiveness of the differently doped

sensors to ethanol as a function of their working temperature

is shown inTable 2 The sensors with an indium concentration

of 5.0 wt.% showed the highest responsiveness when

oper-ated at 250◦C The sensor response to ammonia follows an

involved pattern (seeFig 5) The resistance of the different

sensing layers increased when 10 or 100 ppm of ammonia

were injected into the test chamber, and decreased sharply

when ammonia concentration was increased to 500 ppm

3.2 Structural characterisation

SEM and XRD investigations were used to explain the

experimental results obtained Fig 6 shows the surface

(Fig 6a) and the morphology of the pure WO3 thick film

(Fig 6b) and In doped (5 wt.%) WO3 thick film (Fig 6c)

sensing layers It can be seen that the film surface (Fig 6a)

shows a large quantity of cracks It is assumed that crack

for-mation is related to the high (up to 80◦C/min) temperature

increase rate at the initial stage of firing The average

gran-ule size in the pure WO3 sensing layers was up to 60 nm

(Fig 6b) The morphology of the In doped sensing layer

did not depend on indium concentration The tungsten oxide

layers were nanoparticular with average granule size around

Fig 5 Responses to ammonia of indium-doped WO3 sensors with indium

concentrations of 1.5 wt.% (WIn1), 3.0 wt.% (WIn2) and 5.0 wt.% (WIn3),

operated at 300 ◦C.

Fig 6 SEM images of a pure WO 1 thick film surface (a, b) and indium-doped WO3 thick film with indium concentration of 3 wt.%

70 nm and had uniform granule distribution (Fig 6c) It can

be seen that the morphology of the pure and In doped WO3 sensing layers is almost identical There is just a small dif-ference in the average granule size

XRD data showed that two monoclinic phases (JCPDS cards no 83-0950 and 87-2386) are present in the com-mercial WO3 nanopowders Two monoclinic phases are

described in the P21/n (JCPDS cards no 83-0950) and

Pc (JCPDS cards no 87-2386) space with cell

param-Fig 7 XRD patterns from WO3 nanopowders, and pure (WO 1 ) and

indium-doped WO3 thick films (WIn2 and WIn3).

eters a = 7.3008, b = 7.5389, c = 7.6896, β = 90.892◦ and

a = 5.2771, b = 5.1569, c = 7.666, β = 91.742◦, respectively.

Fig 7 presents the XRD patterns from 2θ = 22◦ to 25◦ of

WO3nanopowder, both pure and In-doped WO3thick films

The XRD patterns contain (2 0 0), (0 2 0) and (0 0 2)

reflec-tions of the monoclinic phase described in the P21/n space

and (1 1 0) reflection of another monoclinic phase After the

firing process, the sensing layers had mainly the first

mon-oclinic phase described in the P21/n space The presence of

metallic indium and indium oxide phase was not detected by

the XRD method On the basis of this result, we can suggest

that In3+ ions were able to diffuse into the tungsten oxide

lattice at the firing temperature used This fact is confirmed

indirectly The basic X-ray lines in the X-ray spectrum show

a shift from the standard position that increases when the

con-centration of indium is increased (seeFig 7) The crystallite

size in the WO3nanopowder, and in pure and In-doped WO3

thick films was evaluated from the diffraction line of (0 0 2)

Table 3 Chemical element distribution in the indium-doped WO3 thick films Indium concentration in

WO3 thick films (wt.%)

Chemical elements

based on the Scherrer equation[13]

D h k l= 0.9λ

β h k l cos(θ h k l) (1) whereλ is wavelength of the incident radiation, β h k lthe full width at half-maximum (FWHM) of the peak in the radiation, andθ h k lthe Bragg angle The average crystallite size of WO3 nanopowder and pure WO3thick film is 30.4 and 45.5 nm, respectively In In-doped WO3 thick films this parameter

is 49.0 nm (1.5 wt.% In), 43.6 nm (3 wt.% In) and 43.0 nm (5 wt.% In)

An analysis of the chemical element distribution in the sensing layers obtained by energy-dispersive X-ray spec-troscopy (EDX) confirmed the absence of chlorine atoms into the sensing layers.Fig 8 shows the chemical element distribution into the In-WO3sensors with an indium concen-tration up to 3 wt.% obtained by EDX It can be seen that tungsten, indium and oxygen atoms are the only constituents

of the sensing layer Estimation of the In/W ratio in the tung-sten oxide films doped with indium performed on the basis of EDX data were in good agreement with the designed indium concentrations of 1.5, 3 and 5 wt.% These results are sum-marised inTable 3

3.3 Discussion

The data obtained from the first set of experiments show that the high response of pure WO3sensing layers (even at

Fig 8 Chemical element distribution in the indium-doped WO3 sensor with indium concentration of 3 wt.%

pure tungsten oxide films at 100 C is the desorption of O2

[14]species NO2replaces the desorbed oxygen species at the

surface of the WO3thick films, which causes a sharp growth

in their resistance Increasing the working temperature of the

sensors changes the equilibrium of the adsorption–desorption

reaction with NO2 The decrease in the number of active

adsorption sites when the temperature of the sensor is raised

causes a reduction of the sensor response to NO2 at

work-ing temperatures higher than 100◦C The addition of glass

frit increases sensor sensitivity because of the presence of

different metal oxide additives

The firing process undergone by pure or In-doped tungsten

oxide films at 600◦C does not dramatically increases particle

size nor promotes the growth of crystallites Furthermore,

particle size of pure or In-doped films is almost identical due

to the fact that indium diffuses into the tungsten oxide lattice

The sensitivity of indium-doped sensors operated at 200

and 300◦C to NO

2 and CO is determined by the defects generated by the inclusion of indium impurities During the

preparation of powder mixtures and the firing process, In3+

ions generated from the breaking of In Cl bonds can diffuse

into the tungsten oxide lattice and form there an additional

number of new defects On the basis of XRD data it can

be derived that these defects are substitutional defects, since

the interplanar spacing of the WO3crystal lattice decreases

when the concentration of indium impurities is increased

Correspondingly, new energetic levels related to the presence

of indium atoms are generated in the band gap of the

In-loaded WO3 These energetic levels are the source of added

support of charge that changes the sensing properties of the

WO3-based sensors

4 Conclusions

The gas sensing properties of pure and indium-doped

nanoparticle WO3 thick films were studied Sensors were

prepared using commercial WO3 nanopowders and

pow-der mixtures with different concentrations of In (1.5, 3.0

and 5.0 wt.%) Their response to different concentrations of

nitrogen dioxide, carbon monoxide, ammonia and ethanol

were investigated It was found that the pure WO3 sensors

responded to nitrogen dioxide even when operated at room

temperature These sensors showed a maximum sensitivity

to NO2 when working at 100◦C The fact that pure WO

3 sensing layers show response to NO2at low temperatures is

due to the small grain size and high surface area of the WO3

nanopowders Indium-doped WO sensors were selective to

for performing the XRD measurements on the WO3 thick films

References

[1] P Shaver, Activated tungsten oxide gas detectors, Appl Phys Lett.

11 (1967) 255–257.

[2] D.G Dwyer, Surface chemistry of gas sensors: H2 S on WO3 films, Sens Actuators B 5 (1991) 155–159.

[3] A.A Tomchenko, V.V Khatko, I.L Emelianov, WO3 thick films gas sensor, Sens Actuators B 46 (1998) 8–14.

[4] A.A Tomchenko, I.L Emelianov, V.V Khatko, Tungsten trioxide-based thick-film NO sensor: design and investigation, Sens Actuators

B 57 (1999) 166–170.

[5] Y.K Chung, M.H Kim, W.S Um, H.S Lee, J.K Song, S.C Choi, K.M Yi, M.J Lee, K.W Chung, Gas sensing properties of WO3 thick film for NO2 gas dependent on process condition, Sens Actu-ators B 60 (1999) 49–56.

[6] D.S Lee, S.D Han, J.S Huh, D.D Lee, Nitrogen oxides-sensing characteristics of WO3-based nanocrystalline thick film gas sensor, Sens Actuators B 60 (1999) 57–63.

[7] I Jimenez, J Arbiol, G Dezanneau, A Cornet, J.R Morante, Crys-talline structure, defects and gas sensor response to NO2 and H2 S

of tungsten trioxide nanopowders, Sens Actuators B 93 (2003) 343–351.

[8] S.-H Wang, T.-C Chou, C.-C Liu, Nano-crystalline tungsten oxide

NO sensor, Sens Actuators B 94 (2003) 475–485.

[9] S Shukla, S Seal, L Ludwig, C Parish, Nanocrystalline indium oxide-doped tin oxide thin film as low temperature hydrogen sensor, Sens Actuators B 97 (2004) 256–265.

[10] S Shukla, L Ludwig, C Parrish, S Seal, Inverse-catalyst-effect observed for nanocrystalline-doped tin oxide sensor at lower operat-ing temperatures, Sens Actuators B 104 (2005) 223–231 [11] A Salehi, M Gholizade, Gas-sensing properties of indium-doped SnO2 thin films with variations in indium concentration, Sens Actu-ators B 89 (2003) 173–179.

[12] J Hub´alek, K Malysz, J Pr´aˇsek, X Vilanova, P Ivanov, E Llobet,

J Brezmes, X Correig, Z Svˇer´ak, Pt-loaded Al2O3 catalytic filters for screen-printed WO3 sensors highly selective to benzene, Sens Actuators B 101 (2004) 277–283.

[13] B.D Cullity, Elements of X-ray Diffraction, 2nd ed., Addison Wes-ley, MA, USA, 1978, p 127.

[14] I.A Myasnikov, V.Ya Sucharev, L.Yu Kupriyanov, S.A Zavialov, Semiconductor Sensors in Physicochemical Investigations, Nauka, Moscow, Russia, 1991, p 327.

Biographies

Viacheslav Khatko graduated in nuclear physics from the Byelorussian

State University (Minsk, Belarus) in 1971 He received his PhD in mate-rial science in 1985 and Dr.Sc in electronic engineering in 2001 In 1975–2003 he worked at the Physical Technical Institute of National Academy of Sciences of Belarus, Minsk, as a researcher, head of the

Laboratory of Electronic Engineering Materials, head of the Thin Film

Materials Department and then as Principal Investigator of the

insti-tute He was Ford SABIT Intern and Ford Visiting Scientist in 1998

and 1999, respectively From April 2003 he is Ram´on y Cajal

profes-sor in the Electronic Engineering Department at the Universitat Rovira

i Virgili (Tarragona, Spain) His current research interests include the

development and application of semiconductor thin and thick film gas

sensors.

Eduard Llobet graduated in telecommunication engineering from the

Universitat Polit`ecnica de Catalunya (UPC), (Barcelona, Spain) in 1991,

and received his PhD in 1997 from the same university During 1998,

he was a visiting fellow at the School of Engineering, University of

Warwick (UK) He is currently an associate professor in the Electronic

Engineering Department at the Universitat Rovira i Virgili (Tarragona,

Spain) His main areas of interest are in the fabrication, and modelling,

of semiconductor chemical sensors and in the application of intelligent

systems to complex odour analysis Dr Llobet is a member of the Institute

of Electrical and Electronic Engineers.

Xavier Vilanova graduated in telecommunication engineering from the

Universitat Polit`ecnica de Catalunya (UPC), (Barcelona, Spain) in 1991,

and received his PhD in 1998 from the same university He is currently

an associate professor in the Electronic Engineering Department at the

Universitat Rovira i Virgili (Tarragona, Spain) His main areas of interest

are in semiconductor chemical sensors modelling and simulation.

Jes ´us Brezmes graduated in telecommunication engineering from the

Universitat Polit`ecnica de Catalunya (UPC), (Barcelona, Spain) in 1993.

Since 1993, he has been a Ph.D student in the Signal Processing and

Communications Department at the same university He has been a lec-turer in the Electronic Engineering Department at the Universitat Rovira

i Virgili (Tarragona, Spain) since 1994 His main area of interest is in the application of signal processing techniques such as neural networks

to chemical sensor arrays for complex aroma analysis.

Jaromir Hubalek graduated in electronic devices and systems from Brno

University of Technology (Czech Republic) in 1996 Since 1996 he has been a PhD student at the Microelectronics Department at Brno University

of Technology and received his PhD in 2003 from the same university His work is focused on precise conductivity detector using planar comb electrodes Since 2000 he has worked in thick-film sensors fabrication at the Electronic Engineering Department at the Universitat Rovira i Virgili (Tarragona, Spain).

Karel Malysz graduated in Materials Engineering at University of

Par-dubice, faculty of Chemistry and Chemical Technology in 2000 Since

2000 he has been a PhD student at the Department of Microelectronics, Brno University of Technology (Brno, Czech republic) His work focuses

on the preparation of new types of semiconductor gas sensors and gas sensing materials.

Xavier Correig graduated in telecommunication engineering from the

Universitat Polit`ecnica de Catalunya (UPC), (Barcelona, Spain) in 1984, and received his PhD in 1988 from the same university He is a full professor of Electronic Technology in the Electronic Engineering Depart-ment at the Universitat Rovira i Virgili (Tarragona, Spain) His research interests include heterojunction semiconductor devices and solid-state gas sensors Dr Correig is a member of the Institute of Electrical and Elec-tronic Engineers.