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In order to enhance the sensitivity of the sensor, the sensing film was fabricated with a porous shape by high temperature heat treatment and the TiO2layer was doped with WO3to improve th

Trang 1

Gas sensing properties of WO 3 doped rutile TiO 2 thick film at high

operating temperature

School of Mechanical Engineering, Yonsei University, Seoul 120-749, Republic of Korea

a r t i c l e i n f o

Article history:

Received 31 March 2009

Received in revised form 29 June 2009

Accepted 29 June 2009

Available online xxxx

PACS:

51.50.+v

Keywords:

WO 3 doped rutile TiO 2

High temperature heat treatment

Grain growth

High temperature gas sensor

a b s t r a c t

A semiconductor gas sensor based on WO3doped TiO2having a rutile phase was fabricated on an Al2O3

substrate The sensing film of the sensor was deposited by using screen printing In order to enhance the sensitivity of the sensor, the sensing film was fabricated with a porous shape by high temperature heat treatment and the TiO2layer was doped with WO3to improve the gas selectivity The surface topography and inner morphological properties of the sensing film were characterized with scanning electron microscopy (SEM), atomic force microscopy (AFM) and X-ray diffraction (XRD) The gas sensing proper-ties of the fabricated sensor were evaluated by detecting NO2and other oxidizing gases (CO, O2and CO2)

at high operating temperature (600 °C)

Ó 2009 Elsevier B.V All rights reserved

1 Introduction

Semiconductor gas sensors can detect oxidizing and reducing

gases by measuring the conductance changes in the surface

phe-nomena in terms of gas adsorption and desorption[1,2] TiO2is

a well known metal oxide semiconductor It can be used for high

temperature gas sensors which are more suitable for controlling

many high temperature technological processes such as the

en-gine’s work of automobiles TiO2exists in several crystalline

mod-ifications, the common forms being anatase and rutile[3] Anatase

is transferred to rutile at above 600 °C[4], so that anatase TiO2

based sensors have relatively low heat treatment temperature

lev-els[5,6] However, low heat treatment temperature can induce

grain growth of the sensing films in sensors operating at high

temperature (500 °C) and decreases the sensors’ sensitivity[7]

It is known a specific surface area of the sensing film is decreased

with increase of grain size[1] So, the grain growth of the sensing

film induces low sensitivity of the sensor On the other hand,

ther-mally stable, rutile TiO2based gas sensors can tolerate high heat

treatment temperature levels Therefore, rutile TiO2 based gas

sensors can detect gases at high operating temperature with no

grain growth Although, the high heat treatment temperature

ensures stable operation of the sensors at high temperature, it

causes grain growth of the sensing films during the heat treat-ment process

Therefore, in this study, WO3doped rutile TiO2was used as the material of the sensing film to reduce the grain growth of the film during the heat treatment process The sensing film was deposited

on an Al2O3substrate by screen printing and was thermally treated

at high temperature (1100 °C) The surface and morphological properties of the fabricated sensor were studied, while its gas sensing properties were investigated by detecting NO2and other oxidizing gases (CO, O2and CO2) at high operating temperature

2 Experimental The sensor was fabricated on an alumina substrate The sensing film was deposited by screen printing on the IDT (interdigitated) electrode A printable paste for screen printing was prepared by using an organic vehicle based onaterpineol The paste consisted

of 16 wt.% nanopowders (average particle size of TiO2: 20 nm,

WO3: 30 nm) and 84 wt.% solvent (aterpineol, ethyl cellulose, dis-persing agent) The 1.5lm thick sensing film was heat treated at

1100 °C for 1.5 h in dry air condition to define a porous shaped sensing film and enhance the thermal stability of the sensor The surface topography of the screen printed sensing film was investigated with an S4800 scanning electron microscope (SEM) operated at 15 keV and an XE150 atomic force microscope (AFM) The structural analysis was carried out by using a D/MAX2500H X-ray diffractometer (XRD)

1567-1739/$ - see front matter Ó 2009 Elsevier B.V All rights reserved.

doi: 10.1016/j.cap.2009.06.053

* Corresponding author Tel.: +82 2 2123 2844.

E-mail address: yjk@yonsei.ac.kr (Y.-J Kim).

Contents lists available atScienceDirect

Current Applied Physics

j o u r n a l h o m e p a g e : w w w e l s e v i e r c o m / l o c a t e / c a p

Please cite this article in press as: S.-E Jo et al., Gas sensing properties of WO doped rutile TiO thick film at high operating temperature, Curr Appl Phys

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Fig 1 SEM images of the sensing films.

Fig 2 AFM images of the sensing films.

Fig 3 XRD patterns of undoped TiO 2 , 11 wt.% WO 3 doped TiO 2 and 15 wt.% WO 3 doped TiO 2

Please cite this article in press as: S.-E Jo et al., Gas sensing properties of WO doped rutile TiO thick film at high operating temperature, Curr Appl Phys

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To characterize the fabricated sensor, it was used to detect NO2

and other oxidizing gases (CO, CO2and O2) The sensor was

oper-ated at 600 °C The ratio of the WO3doping into TiO2was varied

among 0 (undoped), 11 and 15 wt.%

3 Results and discussion

Fig 1 shows SEM images of the sensing films deposited by

screen printing The sensing film exhibited the typical

microstruc-ture of screen printed sensing films A porous sensing film surface

was obtained due to the high heat treatment temperature The

WO3doping did not induce any change

AFM micrographs of the undoped, and 11 wt.% and 15 wt.% WO3

doped TiO2films are shown inFig 2 The AFM micrographs

con-firmed that the WO3doping was not a dominant factor in defining

the porous and rough surface of the sensing films

The crystal structure of the sensing films was revealed in the

XRD patterns shown inFigs 3 and 4 Although, the WO3doping

ef-fect on the surface topography was not clearly evident, the inner

morphological properties of the sensing film were affected by the

WO3doping.Fig 3revealed that the sensing films consisted of

al-most a single phase of rutile TiO2, whileFig 4revealed the (1 1 0)

peak of the rutile TiO2of each film The peaks became broader and

lower with increasing amount of doped WO3 The grain sizes of the

sensing films were calculated by the Scherrer equation(1)

In this study, we used TiO2 nanopowder with a particle size of

20 nm After heat treatment, the grain sizes of the undoped, and

11 wt.% and 15 wt.% WO3 doped TiO2 films were 102.2 nm,

68.1 nm and 62.9 nm, respectively Following the heat treatment,

the grain sizes of the fabricated sensing films increased But, WO3

doping disturbed the grain growth of the sensing films That is,

the WO3 doping clearly affected the grain growth of the TiO2

particles

Fig 5shows the results of NO2(500 ppm) detection at 600 °C

The undoped and two WO3 doped TiO2 sensors exhibited a gas

sensing property similar to that of a p-type, semiconductor gas

sensor The sensitivity to gases is defined as (Ra Rg)/Rg, where

Raand Rg are the steady state resistances of the device in null

(N2) and oxidizing gas, respectively The two WO3doped TiO2

sen-sors had a higher sensitivity to NO2than that of a pure TiO2sensor

Generally, the sensing film with a smaller grain size has a larger

surface area of the sensing film and higher sensitivity[1] There-fore, at 600 °C, the 15 wt.% WO3doped TiO2sensor had the highest sensitivity (145.4) due to its relatively small grain size

Due to its highest sensitivity to NO2, the 15 wt.% WO3 doped TiO2sensor was used to verify the selectivity by detecting three other individual oxidizing gases, CO, O2and CO2, at 600 °C, and a gas mixture (NO2+ oxidizing gases) with CO, O2and CO2 concen-trations of 1000 ppm, 15% and 10%, respectively The detection re-sults of the fabricated sensor for the various oxidizing gases and the gas mixture are shown inFigs 6 and 7 InFig 6, sensitivities

of the sensor (15 wt.% WO3doped TiO2) to CO, O2and CO2were very small And inFig 7, the resistance of the sensor decreased very fast in the NO2condition but exhibited very small changes

in the gas mixtures (NO2+ other oxidizing gases) These sensing re-sults demonstrated the high sensitivity and good selectivity to NO2

of the WO3doped TiO2sensor

The potential energy barrier eVSbetween particles of the sensor

is proportional to Nt,

2Nt

where Ntis the surface density of adsorbed oxygen ions such as O,

O2, O

2 Andere0is the permittivity of the semiconductor, and Nd the volumetric density of the electron donors From Eq (2), we can know that the sensitivity of the sensor is dominated by the number of adsorbed oxygen ions at the sensor surface Because oxi-Fig 4 XRD patterns – (1 1 0) peak of undoped TiO 2 , 11 wt.% WO 3 doped TiO 2 and

15 wt.% WO 3 doped TiO 2

Fig 5 The results of NO 2 (500 ppm) detection at 600 °C.

Fig 6 The results of CO, O 2 and CO 2 detection at 600 °C.

Please cite this article in press as: S.-E Jo et al., Gas sensing properties of WO doped rutile TiO thick film at high operating temperature, Curr Appl Phys

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dizing gases such as NO2are reacted with the oxygen species at the

sensor surface as shown in Eq.(3)

The presence of other metal oxides (WO3) in the lattice of the

TiO2film can cause a change of its electrical property and the inner

morphological properties of the TiO2film, which made the smaller

particle size of the sensor It consequently affected the area of the

sensor surface which related to the number of adsorbed oxygen

ions and the gas sensing properties

Also, we assumed that the high heat treatment temperature of

the sensor induced an electrical interaction between WO3 and

TiO2, which in turn affected the electrical properties of the sensing

film The WO3 may occupy the interstitial sites in TiO2 lattice

according to the following:

WO3TiO!2W00

Since the predominant defect is an oxygen vacancy, this equilibrium

can be described by Eq.(5),

OxO!1

2O2ðgÞ þ 2e

0þ V00

where V00

Ois the oxygen vacancy and OxOis neutral oxygen in lattice

That is, WO3doping induces an increase of the oxygen partial

pres-sure at the sensor surface So, at high working temperature such as

600 °C, there is large number of adsorbed oxygen ions at the

sur-face And it makes the high surface density of adsorbed oxygen ions

and high sensitivity to specific gas such as NO2

4 Summary

A WO3 doped TiO2 sensor, with a sensing film deposited by

screen printing, was fabricated successfully The use of rutile

TiO2 as the sensing film material enabled the heat treatment to

be performed at high temperature, which facilitated the fabrication

of a sensitive and porous shaped sensing film The WO3doping did not exert a dominant effect on the surface topography but it did af-fect the inner morphological properties of the TiO2 film The

15 wt.% WO3 doped TiO2 sensor showed the best sensitivity to

NO2at 600 °C The proposed sensor had good sensitivity and selec-tivity to NO2, in comparison with its sensitivity to other oxidizing gases such as CO, O2, and CO2

Acknowledgements This work was supported in part by the Information Technology Research Center support program (IITA-2005-C1090-0592-0012), and in part by Korea Science and Engineering Foundation (KOSEF) Grant funded by the Korea government (MEST) (2008-8-1253)

References [1] G Korotcenkov, Materials Science and Engineering B 139 (2007) 1–23 [2] C.O Park, S.A Akbar, Journal of Materials Science 38 (2003) 4611–4637.

[3] L Francioso, D.S Presicce, M Epifani, P Siciliano, A Ficarella, Sensors and Actuators B 107 (2005) 563–571.

[4] S.J Kim, G.H Chang, Y.C Jin, G.R Jheong, Journal of the Korean Society for Heat Treatment 7 (1) (1994) 11–16.

[5] Ken-ichi Shimizu, Kohichi Kashiwagi, Hiroyuki Nishiyama, Shiro Kakimoto, Satoshi Sugaya, Hitoshi Yokoi, Atsushi Satsuma, Sensors and Actuators B 130 (2008) 707–712.

[6] A Wisitsoraat, E Comini, G Sberveglieri, W Wlodarski, A Tuantranont, in: The 2nd IEEE International Conference on Nano/Micro Engineered and Molecular Systems, 16–19th January 2007, Bangkok, Thailand.

[7] S.S Bhoga, K Singh, Ionics 13 (2007) 417–427.

Fig 7 The results of gas mixture detection at 600 °C.

Please cite this article in press as: S.-E Jo et al., Gas sensing properties of WO doped rutile TiO thick film at high operating temperature, Curr Appl Phys

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