sensitivity properties of a novel no2 gas sensor based on mesoporous wo3 thin film

Hona,∗ aDepartment of Materials Science and Engineering, National Cheng Kung University, 1 Ta-Hsueh Road, Tainan 70101, Taiwan, ROC bNational Nano Device Laboratories, 1001-1 Ta-Hsueh Ro

Sensitivity properties of a novel NO 2 gas sensor based

L.G Teoha, Y.M Hona, J Shiehb, W.H Laia, M.H Hona,∗

aDepartment of Materials Science and Engineering, National Cheng Kung University, 1 Ta-Hsueh Road, Tainan 70101, Taiwan, ROC

bNational Nano Device Laboratories, 1001-1 Ta-Hsueh Road, Hsinchu 30050, Taiwan, ROC

Received 19 December 2002; received in revised form 20 May 2003; accepted 27 May 2003

Abstract

Mesoporous WO3 thin films micro-gas sensor was fabricated and the NO2 gas-sensing as well as electrical properties have been investigated The film had nano-sized grains, porous structure with a relative surface area of 143 m2/g as calcined at 250◦C Upon exposure

to NO2, the electrical resistance of a semiconducting mesoporous WO3thin films is found to dramatically increase The sensitivity of mesoporous WO3thin film sensors is substantially higher than that from other reports In addition, the mesoporous WO3thin film sensor calcined at 250◦C and operated at 35◦C shows an excellent sensitivity of 23, as we know it is unique NO2gas sensor which has the

sensitivity at such a low temperature

© 2003 Elsevier B.V All rights reserved

Keywords: Mesoporous WO3; NO2 gas sensor; Sol–gel process

1 Introduction

The detection of NO2is important for monitoring

environ-mental pollution resulting from combustion or automotive

emissions[1] Existing gas sensor materials include

semi-conducting metal oxides[1], silicon[2,3]and organic

mate-rials[4,5] Semiconducting metal oxides such as WO3 and

SnO2had been widely used for NO2detection[1,6] These

sensors have to operate at 200–500◦C in order to improve

the sensitivity by enhancing the chemical reaction between

gas and the sensor material [7,8] Obviously, it would be

desirable for many applications if the sensor could operate

at temperatures <100◦C or even at room temperature,

es-pecially for battery-operated devices Recently it also has

been reported that ZrO2–SnO2 [9]and ZnO[10]materials

can be used as H2S and NH3gas sensors at room

temper-ature, respectively, but their sensitivity was low WO3 was

reported to exhibit promising electrical and optical

proper-ties for various applications like efficient photolysis,

elec-trochromic devices, selective catalysts and gas sensors[6]

WO3is an n-type semiconductor whose electron

concen-tration is determined mainly by the concenconcen-tration of

stoi-chiometric defects such as oxygen vacancy like other metal

oxide semiconductors The first work on the feasibility of

∗Corresponding author.

E-mail address: mhhon@mail.ncku.edu.tw (M.H Hon).

WO3 thin films as a gas sensor was reported by Shaver [11]who observed that the conductivity of WO3thin films changed greatly upon the exposure to the H2 ambient Following this pioneering work, many works have been per-formed on the structural and electrical properties and sens-ing characteristics of WO3 thin films These sensors have been reported to have good selectivity for low concentration

NOxgas[12] In this study, we developed a novel NO2gas sensors based on mesoporous WO3thin film to detect small concentration of NO2at low operating temperatures

Li and Kawi[13] have shown that a linear relationship was found between the surface areas of SnO2 sensors and their sensitivities to 500 ppm of H2 Accordingly, meso-porous WO3 with a higher surface area provides more surface adsorption sites for the reaction of NO2gas, which

is beneficial to the operating temperature and sensitivity of the sensor Mesoporous materials are generally prepared

by amphiphilic self-assembling surfactants as templates [14,15] In this study, the mesoporous WO3thin film sensors synthesized by sol–gel process using triblock copolymer

as the template were reported X-ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM), Brunauer–Emmett–Teller (BET) and conductivity measurements were used to characterize the microstructure and electrical properties of mesoporous WO3 gas-sensing films that were deposited by dipping on Al2O3 substrate

0925-4005/$ – see front matter © 2003 Elsevier B.V All rights reserved.

doi:10.1016/S0925-4005(03)00528-8

2 Experimental

Poly(alkylene oxide) block copolymer (BASF Pluronic

EO100PO64EO100, F127) was used as a template material

About 0.5 g of F127 copolymer was dissolved in 10 g of

ethanol Then 0.01 mole of the anhydrous tungsten chloride

precursor, WCl6(Aldrich 99.9%), was added into the F127

ethanol solution with vigorously stirring for 1 h The

result-ing sol solution was gelled in an open Petri dish at 60◦C

in air Alternatively, the sol solution can be used to prepare

thin films on Al2O3substrate that was coated with Pt

elec-trode by dip coating The thin films can be dried within

sev-eral hours at 60◦C The as-made bulk samples or thin films

were calcined at 250◦C for 5 h and then washed by ethanol

to remove the residual block copolymer

X-ray powder diffraction (XRD) patterns were obtained

on a Rigaku D/max-X-diffractometer using Cu K␣ radiation

with Ni filter Transmission electron microscopy (TEM)

studies were carried out on a Hitachi Model HF-2000

elec-tron microscope operating at 200 keV The samples for

TEM were prepared by directly dispersing the fine powders

of the product onto 200 mesh Cu grids The morphology of

mesoporous WO3films was observed by scanning electron

microscope (SEM, Philips XL-40 FEG) The nitrogen

ad-sorption and dead-sorption isotherms at 77 K were measured

using a Micromeritics ASAP 2010 system after the samples

were vacuum-dried at 150◦C for 10 h in N

2 atmosphere

∆

∆

∆

∆

∗

∗

∗

∗

∗

∗ Substrate

∆

∗

Fig 1 X-ray diffraction pattern for a mesoporous WO3 thin film calcined at 250 ◦C for 5 h.

Brunauer–Emmett–Teller (BET) surface areas were

esti-mated over a relative pressure (P/P0) range from 0 to 1.0 Pore size distribution was obtained from the analysis of the adsorption branch of the isotherms using the Barrett– Joyner–Halenda (BJH) model The pore volume was taken

at theP/P0 = 0.983 signal point.

The resistance of the films was obtained by measuring the current through the film at a constant voltage of 1 V and recorded by a multimeter (HP 3458 A) The samples under test were placed in a quartz chamber (85 cm3) and exposed to 3 ppm NO2gas and 4000 ppm H2, respectively Gas-sensing properties of the films were studied at various

operating temperatures Tg in the range of 35◦C < Tg <

100◦C The sensitivity is defined as Rg/Ra, where Rg and Ra

are the electric resistance in test gas and air, respectively

3 Results and discussion

3.1 Microstructure characterizations

Fig 1shows the XRD pattern of mesoporous WO3 thin films calcined at 250◦C for 5 h indicating that this

crys-tallographic nucleation actually occurs during the calcina-tion, but is limited to formation of nanocrystallite domains The diffraction peaks of the WO3 thin films are assigned based on monoclinic structure (JCPDS card no 05-0363)

Fig 2 SEM micrograph of mesoporous WO3 thin film calcined at 250 ◦C

for 5 h.

After employing Scherrer’s formula, the calculated grain

size of WO3is approximately 3.8 nm These grains contact

contributes to the gas-sensing properties of the mesoporous

WO3 films (the smaller grain size increases gas sensitivity

since the diameter is comparable with or less than the space

charge region of the grain)

Fig 2shows the SEM micrographs of the WO3thin films

calcined at 250◦C for 5 h The sample exhibits porous

struc-ture with a spherical powder of approximately 1.5␮m It

means that such a structure of film is likely to facilitate the

adsorption process of NO2 molecules because of the

cap-illary pore and large surface area This implies the

conclu-sion that this type of film will offer a good sensitivity to

NO2gas

The morphology of the mesoporous WO3 thin film was

characterized by TEM Fig 3a shows a bright field TEM

image, in which the pores with a mean size of∼5 nm can

be clearly observed The size of the mesopores estimated by

TEM is in agreement with the values determined from the

adsorption data (BET) Selected-area electron diffraction

patterns recorded on mesoporous WO3that is characteristic

of diffuse electron diffraction rings demonstrate that the

walls of our material are made up of nanocrystallite This is

also supported by the dark field TEM image (Fig 3b), which

reveals that the framework consists of nanocrystals (the

bright spots in the image correspond to WO3nanocrystals,

∼3 nm) and agree with the result of grain size determination

obtained from XRD analyses The results lead to conclude

that the crystallized WO3 is essential for obtaining high

sensitivity or expected to afford higher sensitivity toward

gas-sensing reactions The pore size and the wall thickness

can be estimated from TEM in 5.4 and 1.8 nm,

respec-tively Nitrogen adsorption–desorption isotherms exhibiting

a type IV curve is shown inFig 4, which is characteristic

of mesoporous WO3 [16] Barrett–Joyner–Halenda (BJH)

analyses show that the calcined mesoporous WO3exhibits

mean pore size of 5 nm (Fig 4 inset) From the absolute

adsorption, we can calculate a specific surface of 143 m2/g

This underlines that most pores are really accessible from

Fig 3 TEM images of mesoporous WO3 thin film calcined at 250 ◦C for

5 h: (a) bright field TEM image; (b) dark field TEM image obtained on the same area of (a); (a) inset: selected-area electron diffraction pattern recorded on the sample.

the outside and the pore system is fully interconnected (from adsorption and desorption lines of N2)

3.2 Gas-sensing properties

In order to check the sensitivity of WO3 sensors for the concentration of 3 ppm NO2, WO3sensors were maintained

at various temperatures from 35 to 100◦C.Fig 5shows that

WO3 sensors respond to turning-on and turning-off NO2 flow by the reversible changes of electrical resistance The resistance values in air decrease with a rise in operating temperature, which is a typical characteristic of ceramics When the NO2was introduced into the test chamber, the re-sistance of sensor increased and soon afterwards it became saturated When the gas was turning-off, the resistance of

Pore diameter,(Å)

20 40 60 80 100

Relativep ressure (P/P0)

0 0.4 0.8 1.2 1.6

Fig 4 Nitrogen adsorption( +)–desorption(䊊) isotherms and BJH pore size distribution curves for mesoporous WO3 calcined at 250 ◦C for 5 h.

the sensor decreased The time response of WO3 sensors,

which shows good sensitivity to NO2, is shown inFig 6

After initial resistance was stabilized, NO2was injected into

the closed chamber in the batch system and vented the gas

after being maintained for 5 min The sensors with

operat-ing temperatures >70◦C and operating temperatures<50◦C

have a 90% response time of 1–2 min and above 10 min,

re-spectively.Fig 7illustrates NO2 gas sensitivities of WO3

sensors to 3 ppm NO2from 35 to 100◦C It is evident that

the films are able to detect 3 ppm of NO2in air at low

tem-perature The results show the systematic changes of WO3

conductivity with decreasing operating temperatures More

importantly, the results clearly show that a higher surface

area WO3sensor has a much better sensitivity response to a

low concentration gas For comparison, it was reported that

a metal oxide sensor (thick film type WO3by screen

print-ing) operated at 100◦C for detecting 100 ppm of NO

2with

a sensitivity of ∼200 [17] and a high-performance metal

oxide sensor (Cd-doped SnO2) operated at 250◦C for

de-tecting 100 ppm of NO2with a sensitivity of∼300[4,18]

Thus, the mesoporous WO3sensors have the advantage of

100◦C temperature operation for detecting 3 ppm with

sen-sitivity up to 226 over these materials

The experiment on the selectivity of the mesoporous gas

sensor was carried out by monitoring the electrical

resis-tance change in H2 atmosphere, as shown inFig 5 It can

be seen that the sensor operating at 100◦C and 4000 ppm of

H2 exhibits a sensitivity of∼3 Although the sensitivity is

much smaller than that of NO2, the opposite response in re-sistance (NO2 increases the resistance, while H2 decreases the resistance) demonstrates that the mesoporous gas sen-sor has the selectivity to distinguish oxidizing and reduc-ing gases The stability of the sensreduc-ing characteristics was examined several times in a week It was found that the initial resistance was approximately maintained, but with

a slightly increased resistance at saturation For example, the sensitivity change was about 10% over this period This indicates that the long-term stability properties should be improved

The most important factors that influence the WO3sensor characteristics are probably microstructure and surface area The films exhibiting a porous structure have a large fraction

of atoms residing at surfaces and interfaces between the pores, which suggests that the microstructure of the films is suitable for gas-sensing purposes In the other words, it can

be said that the high sensitivity of a mesoporous sensor can

be attributed to the full exposure of surface adsorption sites

to chemical environments

As for the microstructure, maintaining smaller crystal sizes can improve device performance The mesoporous WO3thin film contains crystallites∼3.8 nm in diameter

em-bedded in an amorphous matrix Semiconductor gas sensor

Time (sec)

0.0E+000 4.0E+008 8.0E+008 1.2E+009

↓

On

←Off

100 oC

70 oC

50 oC

35 oC

NO2

NO2

(a)

Time (sec)

0.0E+000 2.0E+005 4.0E+005 6.0E+005

H2 off

↑ 100oC

(b)

Fig 5 Dependence of electrical resistance of WO3 thin films upon operating temperatures of (a) 35, 50, 70 and 100 ◦C for 3 ppm NO2 gas and (b)

100 ◦C for 4000 ppm H2 gas.

Temperature (oC) 0

10

20

Fig 6 Time response of WO3 thin film sensor.

Operating temperature ( 0C)

0 50 100 150 200 250

Fig 7 Sensitivity of mesoporous WO3 thin film upon operating temper-atures from 35 to 100 ◦C.

typically utilizes the gas-induced variations in potential

barrier height at grain boundaries (i.e changes in thickness

of the space charge layer), and it is well known that the

gas sensitivity increases with decreasing the particle size

[19,20] It is also noted that earlier work by Xu et al.[21]

stated that the gas sensitivity is controlled by a grain size

effect for WO3 crystallites smaller than 33 nm; this result

is consistent with a theoretical model by Wang et al.[22]

Based on these fundamental aspects, a possible NO2-sensing

mechanism of the present WO3is depicted as the following

The molecular NO2has an unpaired electron and is known

as a strong oxidizer Upon NO2adsorption, charge transfer is

likely to occur from mesoporous WO3to NO2because of the

electron-withdrawing power of the NO2molecules The NO2

ions adsorbed at low temperatures on oxide semiconductor

surfaces are thought to be ONO− (nitrito type adsorbates)

and dissociate into nitrosyl type adsorbates (NO+, NO−)

[23] This enables to conclude that the normal response of

sensor for NO2might originate from the superior number of

NO+absorbates than NO−adsorbates, even at room

temper-ature Consequently, the electron transfer to surface species

in connection with NO2 chemisorption creates Schottky

energy barrier at the surface yielding a large resistance of

the film It is clear that the response is related to a catalytic

reaction of WO3with the adsorbed NO2ions A release of

electrons from the surface species increases the height of the

surface barrier, thereby resulting in an increase of the film

resistance In a model of Wang et al [22], a small grain

size, such as in the deposited nanocrystalline WO3 films

after sintering at 480◦C, improves the gas sensitivity In

summary, for polycrystalline conductors, grain boundaries

contribute most of the resistance The surface resistivity of

an oxide crystal depends on the electron concentration near

the surface, which in turn is affected by the nature of the

chemisorbed species Theoretically, the smaller the crystal

size, the greater the sensitivity of overall resistance to the

surrounding atmosphere

In addition, the films also show a sensitivity of 23 at

35◦C As we know it is unique for the WO

3 film that has the sensitivity to NO2at such a low temperature Therefore,

it can be assumed that the mesoporous WO3film exhibits a

high sensitivity at a low temperature to 3 ppm NO2

4 Conclusions

This study has shown that mesoporous WO3 thin films

with unique microstructure lead to excellent sensing

prop-erties upon exposure to low concentration of NO2 in air

at low temperatures and enabled the selective detection of

NO2 and H2 gases A high surface area and small

crystal-lites present in the mesoporous WO3 films are the factors

contributing to this behavior Apart from small grain sizes,

the main feature of mesoporous WO3 thin film sensors is

that they operate at low temperatures and low concentration

of NO with sensitivity as high as 226 Thus, mesoporous

WO3thin film should be promising for advanced miniatur-ized chemical sensors

Acknowledgements

This work was financially supported by the National Sci-ence Council of Taiwan, ROC, grant No NSC 90-2216-E-006-064, which is gratefully acknowledged

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Biographies

Lay Gaik Teoh received her BS and MS degrees of Materials Science

and Engineering from National Cheng Kung University, Tainan, Taiwan

in 1997 and 1999, respectively She has been a PhD candidate at National

Cheng Kung University, Tainan, Taiwan, 1999 Her major research has

related to mesoporous materials, semiconductor gas sensor, and PVD

Ba-Ti-Sn-O system thin films.

Yi Ming Hon received his BS, MS and PhD degrees of Materials Science

and Engineering from National Cheng Kung University, Tainan, Taiwan

in 1995, 1996 and 2001, respectively He is now a postdoctoral fellow

at National Cheng Kung University, Tainan, Taiwan His major research has related to mesoporous materials, and battery materials.

Jiann Shieh received his BS, MS and PhD degrees of Materials Science

and Engineering from National Cheng Kung University, Tainan, Taiwan

in 1995, 1997 and 2002, respectively He is now an associate researcher at National Nano Device laboratories Hsinchu, Taiwan His major research has related to PECVD Ti-Al-C-N system nanocomposite thin films, semi-conductor gas sensor, mesoporous materials, nanocrystal, and nanowire materials.

Wei Hao Lai received his BS and MS degrees of Materials Science and

En-gineering from National Cheng Kung University, Tainan, Taiwan in 2000 and 2001, respectively He has been a PhD candidate at National Cheng Kung University, Tainan, Taiwan, 2001 His major research has related

to mesoporous materials, semiconductor gas sensor, and nanomaterials.

Min Hsiung Hon is a professor in the Department of Materials

Sci-ence and Engineering in National Cheng Kung University, Tainan, Tai-wan His research interest includes thin film deposition, battery materials, gas sensor, lead-free solder, biomaterials, nanomaterials, and mesoporous materials.